Diaries of Labour peer reveal pre-election absence of coalition plans even though polls showed it as best possibility for Brown

Labour went into the 2010 general election with no plans in place of how to form a coalition with the Liberal Democrats, even though the polls showed that such an outcome was the best possible outcome for Gordon Brown's administration.

The absence of any plans emerges in the diaries of Lord Adonis, the Labour peer and one of the key figures in the failed talks between Labour and the Lib Dems.

Adonis reveals that Brown's campaign rebuffed a pre-election suggestion by the then prime minister that Labour stand down in 30 Lib-Con marginal seats in a gesture designed to increase the Liberal Democrat force in the Commons, and forge a common ideological alliance against the Tories.

The absence of planning is surprising as Lord Mandelson, the then business secretary, had been involved with extensive Lib-Lab talks under the Blair administration. The Adonis diaries reveal that Mandelson's one attempt to discuss the potential post-election landscape – a chance discussion with Lord Ashdown – led to the former Liberal Democrat leader doing everything possible to avoid him.

Brown also admitted after the 2010 defeat that he told the Liberal Democrat leader, Nick Clegg, he had not built up any personal relationship with him ahead of the election.

Adonis is one of the relatively small group of Labour politicians who, after the election result, thought it would have been possible to form a rainbow coalition with the Liberal Democrats and some nationalists in order to keep the Conservatives – the largest party – out of government.

In his diaries, currently serialised in the Times, Adonis suggests the first serious discussion about a possible Liberal Democrat deal was not held until he broached the subject with Mandelson on polling day.

According to Adonis's account, Mandelson was not just sceptical but hostile, saying: "The Tories are going to win. That's been the position for months – two years – now, and it's still what's going to happen one way or another. And I tell you, if the country wakes up on Saturday and Labour is still there, there will be a wave of national revulsion."

Adonis says Mandelson told him: "As for your Lib Dems, let me tell you about Paddy [Ashdown]. He was on the same train as Gordon and me, returning from the Newcastle rally last Saturday. So I went and plonked myself down next to him. He couldn't get away from me fast enough. It was as if I was a leper. He suddenly needed to go to the lavatory; to speak on his mobile; to do anything but speak to me or be seen with me. That's what your Lib Dems think about talking to Gordon."

The Adonis diaries also appear to show that no internal Labour discussion was held about the possibility of Brown agreeing to stand down quickly if no party had won an overall majority and the possibility of a Lib-Lab deal was live. That issue – a potential roadblock to co-operation – was not resolved until Sunday, two days after the results became clear on Friday morning.

Adonis also claims that Liberal Democrats – probably Ashdown – deliberately briefed the BBC that a first conversation between Clegg and Brown at 5pm on the Friday had been a disaster, with Brown hectoring the Lib Dem leader.

Adonis insists the conversation had been workmanlike, with Brown at one point telling Clegg: "I've been reading your manifesto and I've written two papers on your policy positions and how we can work together. I can get them to you tonight if you like."

Adonis writes: "Nick seemed surprised at this but said: 'I'd really like to see your note.'"

The diaries go on to show how Brown, on the Monday after the election, was forced to offer his resignation when the Liberal Democrats rejected as insufficient promises to offer a free hand on Europe and a referendum on constitutional reform in which the coalition government would advocate a yes vote.