The DUP turned down an offer from Theresa May to form a coalition government in the aftermath of last year's snap General Election disaster, a new book has claimed.
Had it been accepted, the move would have seen DUP ministers in the Cabinet.
However, Arlene Foster's party instead entered into a 'confidence and supply' agreement with the Tories, backing the Government on key votes in exchange for funding for Northern Ireland.
The claims - which have been met by silence from both the DUP and No. 10 - are outlined in the book, The British General Election 2017, excerpts of which were contained in an article published by the Huffington Post.
Written by academics Philip Cowley and Dennis Kavanagh, the book asserts how the Conservatives tried to woo the Northern Ireland party as part of efforts by the Prime Minister and her allies to prevent Labour leader Jeremy Corbyn from entering Downing Street.
Based on interviews with various sources, the writers insist significant legwork - which started the day after the Conservatives' dismal performance denied Mrs May a majority - had gone into the offer on a full coalition.
However, opposition from some of Mrs May's own MPs to the DUP's positions on issues such as abortion and same-sex marriage led to her deciding to withdraw the offer.
But chief whip Gavin Williamson is said to have warned the PM that to withdraw the offer would be counterproductive, as negotiations were at a sensitive stage.
"You've got to show them that you're serious and not to patronise them," one Tory source told the authors.
The PM's dilemma was effectively resolved when the DUP made it clear that it would in fact prefer the current "more relaxed" arrangement, whereby Mrs Foster would ensure her 10 MPs would back the Government in key votes - such as Budgets and the Queen's speech - in return for a £1bn cash boost for the province. Ironically, the talks took so long that the Queen's speech was delayed.
Talks had started after Mr Williamson had boarded a plane to Northern Ireland at RAF Northolt the day after the election, still dressed in the suit he had worn at his election count.
He met Sir Jeffrey Donaldson at the Stormont Hotel for talks. But the next day they moved to a hotel in Sir Jeffrey's Lagan Valley constituency to avoid prying eyes.
Before Mr Williamson left for Belfast, the book reveals that "there had been a long meeting in the Cabinet room to discuss what could be offered to the DUP".
"The room was hot, there were too many participants and everyone - including the Prime Minister - was exhausted.
"Eventually, it was agreed that the DUP could be offered a confidence-and-supply arrangement, by which they would support the Government on key votes, or a full coalition, in which they would also sit in the Government as ministers."
After nearly two weeks, the long-awaited deal was finally signed by Sir Jeffrey and Mr Williamson, in front of Mrs May and Mrs Foster. Both No. 10 and the DUP remained tight-lipped on the claims yesterday.