The reasons Dafydd Elis-Thomas left Plaid Cymru have been laid out in a new book detailing the life of the late politician who was described by its author as "one of the most significant Welsh political figures of recent decades".

Author Aled Eirug was friends with Dafydd Elis-Thomas and had known him for more than 50 years. He said he agreed to his friend's request to write the book on one condition - that Dafydd El, as he was best known, didn't see it until it was published.

That transpired to be true, albeit not for the reason intended, as the book has been published seven months after Dafydd El's death in February this year.

Mr Eirug says his friend would have welcomed the frank and often robust criticism some of his contemporaries make of him within the book, being published in the coming days. He would not, he said, have wanted his story sugar-coating, or rewriting, the author explained.

The book is based on a series of interviews with Dafydd El himself, conducted in the autumn of 2020, but also interviews with around 70 other figures including fellow major players in Welsh politics.

The book details his younger days and his election as a young MP right through to his death. It also explains what happened behind closed doors in the lead-up to him leaving the party to which he had given so much.

After spending 11 years as the Senedd's presiding officer Dafydd El put his name forward in the party's leadership contest in 2012, standing against Leanne Wood and Elin Jones, but he was knocked out in the first round.

While he said he had only entered the contest to "show that I was still around" the book describes how he was "bruised and personally disappointed" by his loss and "swiftly became dissatisfied with [eventual winner] Leanne Wood".

Mr Eirug writes: "He had underestimated Leanne Wood and had not given her due regard as a serious politician."

Dafydd El rejoined his party group as a "comparatively minor member of the Plaid Cymru group" under Ms Wood. There was, Mr Eirug writes, "inevitable conflict with the party's leadership".

Ms Wood told the author that in the build-up to the 2016 Assembly election, he was a "wrecker, a spoiler...whatever we were trying to achieve, he was trying to undermine it".

She describes his attitude to her personally as being down to "some sort of misogyny".

Ms Wood said before 2012 they had a "warm relationship".

"We got on well throughout that election but after the result things changed dramatically," she says. "It was such a big change - he didn't want anything to do with me and it became an obstacle to what we were trying to do.

"I didn't understand his change in attitude. I put it down to some sort of misogyny, not being able to accept that a younger woman had defeated this well-established, well-known politician and he seemed to be irked by it. But I never understood it and we never discussed it."

In July 2015 a constituency meeting was held in Dwyfor Meirionnydd to discuss the conduct of their local Assembly member who was facing internal disciplinary action for publicly questioning Plaid's election priorities

Dafydd El had the support of his constituency and a deal was done between the party executive and the AM but Mr Eirug writes it was a "bruising meeting" and "further poisoned the relationship between him and the party centrally". For our free daily briefing on the biggest issues facing the nation sign up to the Wales Matters newsletter here.

Then party chair Dafydd Trystan described how there was a "strange period" after Dafydd El survived a vote of the party's executive.

He says: "There was a contrast between Leanne's method of running the party and Dafydd's wish to constantly attract attention to himself. He felt that the group in the Assembly was beneath him."

Others believe he should have been dismissed from the party then.

Claire Howells, a political consultant hired by the party, describes how she believed he should have been sacked earlier due to undermining Ms Wood.

"He systematically undermined Leanne throughout the period 2013 to 2016," she says.

But the book admits that during the 2016 Assembly - as was - campaign Dafydd El disagreed publicly with his party, admitting he deliberately "provoked" them by announcing that he was supporting the Labour candidate in the police and crime commissioner election in 2016.

However the book's subject's memories of the period between 2012 and 2016 differed.

"There was no intentional falling out but if I was asked I would voice my views. I had support in the constituency. The problem with [Leanne Wood] was that she didn't understand the job - she had no interest in anywhere beyond the south Wales valleys. Plaid wasn't very successful under her leadership," he said.

The book's author wrote that "Dafydd Elis continued to disparage her leadership and during the 2015 general election attacked the party's election strategy".

The book details how even before the 2016 election Dafydd El had been speaking to then Labour First Minister Carwyn Jones about leaving the party in a "light-hearted" way.

Carwyn Jones, now Lord Jones, said: "I started talking to him before the 2016 election, in a light-hearted way - not too seriously - saying that we could work together. I didn't think he would leave Plaid then, because he was still a candidate, and I didn't expect he'd join Labour. I told him that if he left Plaid there would be a place for him in government but there was no question of agreeing anything before the election."

After Carwyn Jones was named First Minister he told Dafydd El there was a place for him in the administration.

"I was trying to tempt him over - of course I was - it was a political opportunity," he tells the author.

In October that year Dafydd El quit Plaid Cymru. The book details a meeting when he made it clear he was disappointed that Plaid's Assembly group had failed to work with Labour, announcing he would leave and sit as an independent. That left the members "stunned".

After 56 years, having left the party, he headed off straight from the meeting to a family party to mark his 70th birthday.

It caused bitterness and upset, the book describes, with MP Liz Saville-Roberts accusing him of "betraying" the national movement.

"The final act that persuaded Dafydd Elis-Thomas to leave Plaid Cymru was the decision by Leanne Wood, encouraged by Adam Price amongst others, to put herself forward as the First Minister, even though she did not have a majority of the seats in the Assembly," the book says.

Ms Wood told the book's author: "I only remember the grief he caused the party. In the early Cymru especially after the 1979 referendum disaster result he did a lot of good work on where Plaid stood on the key questions of the day such as the Miners' Strike. Unfortunately a lot of that work has been undone by his undermining of Plaid Cymru for no good reason other than a personal sense of grievance after the 2012 leadership election.

"Also sometimes people have felt he has trolled the party, winding people up on purpose, putting things out there knowing that members would be angry, especially for his support for the royal family, and why? To what end? I genuinely don't understand, don't know where he's coming from, is it for fun?"

Referencing him joining the government as culture minister she asked: "As for joining the government, why did he do that? What did he achieve?"

In 2020 Dafydd El announced live on BBC radio he would retire and not seek election and by 2021. He then all but withdrew from public life.

However in 2023 Dafydd El did try to rejoin Plaid Cymru. But he said an internal panel set up to determine his position was "hostile". An email sent to him detailing the process was "less than encouraging" and the book explains: "Elis-Thomas decided not to engage with the panel and its disciplinary process and to attend what seemed to him be a kangaroo court held on a remote video link."

There are lighter stories, too, such as how the family adjusted to his life as a new MP in London.

His first wife, Elen, detailed the family's trip to London in order for him to take his oath as an MP.

"I hadn't been on a motorway before, never mind going to London," she says.

And it details the relationships he had with Gwynfor Evans and Dafydd Wigley.

Having been elected aged 27 Dafydd El was the youngest member of the House of Commons and the book details how he ended up living with Gwynfor Evans who was then 61. Dafydd Wigley had been offered a room in a flat of businessman Brian Morgan Edwards and took the offer given he "wasn't really in a hurry to share a flat with Gwynfor".

"Dafydd Elis, however, found himself compelled to share a flat with Gwynfor for the following five years...This was an incongruous match and in addition to their wildly different personalities Dafydd Elis also privately viewed Gwynfor's leadership of the party as overly patrician and autocratic and disagreed with his extreme pacifism and innate conservatism... Dafydd recalled taking Gwynfor to his first experience of a wine bar," the book reads.

It details the times he disagreed with Plaid Cymru and how he privately expressed doubts as to whether he would consider himself a a nationalist.

While walking with his friend Gwynn Matthews in 1974 he told him: "I'm not a nationalist anymore. I feel like a priest who has lost his faith."

The politician described how his opposition to membership of the European Economic Community was his "biggest political mistake".

"I'm not really keen to defend myself on any of that, it was wrong. I shouldn't have done what I did," he said.

Speaking of the failed 1979 devolution campaign Dafydd El described it as "a bloody awful campaign. Very badly run, and organised, and led. It's a very good example of a not properly informed electorate being asked a question which they don't understand. But it's more than that. Obviously, with a margin of four to one against, it's clear that the people of Wales didn't want it at the time."

The period after that, until 1992 was a "momentous time" for him both professionally and personally.

"The disappointment of the 1979 Referendum loss depressed many in the national movement but fired Dafydd Elis up to force Plaid Cymru to confront its own contradictions," Mr Eirug writes.

It details the breakdown of any relationship with Gwynfor Evans after he threatened to go on hunger strike in 1980. In 1980 the then-Plaid Cymru MP for Carmarthen said he would fast to death if the government did not provide a Welsh-language TV service.

Dafydd El said: "Gwynfor was going around the country on a suicide mission basically. He was holding all these public meetings and stirring people up. I really shouldn't say this but I lost respect for him during that period. If he was younger I would have said it was an attention-seeking attitude. This was some of my difficulty in my relationship with him over the years. I never knew, really, where he was coming from or where he was trying to go.

"I don't think Gwynfor thought about Plaid. He thought about some ageing histrionics he could make for himself."

The theme of disagreements between "the two Dafydds" runs throughout the book, particularly when it emerged Dafydd El had been having meetings with Sinn Fein representatives and held a press conference to support the election campaign of Bobby Sands - an IRA commanding officer in the Maze prison.

Dafydd Wigley considered it a "controversial intervention".

The book describes the Senedd building in Cardiff Bay as a "physical legacy" of the politician. He backed the design and, despite trials and tribulations including it going nearly six times over the original 2000 budget, his "love and regard" for both the design and building "was unquestionable".

His resilience in persevering with the £70m building as we know it today was praised by Labour's Edwina Hart and credited by architect Ivan Harbour who said the project would not have been completed at all but for Dafydd El's support.

His insistence it would be called the "Senedd" was more controversial, resulting in a row with Labour's Leighton Andrews, and even saw the Queen change her speech to avoid controversy.

He was 78 when he died with the book explaining how he had become increasingly frail and by the end of 2024 was finding it difficult to swallow his food. He had been referred for urgent tests but before they were completed he died.

His funeral was held at Llandaff Cathedral and attended by political characters and figures from Welsh cultures across the board.