Anthony Seldon: How Brown and Clegg let it slip

Brown's decision to offer his own head stunned Clegg and made him realise, for the first time, that Brown was serious about trying to make a Lib-Lab pactwork

Much has been written about the dramatic negotiations that took place between the Liberal Democrats and the Conservatives and Labour in the immediate aftermath of the indecisive election of May. Yet Gordon Brown's role in these momentous days is in danger of being badly misrepresented. His role needs to be reappraised if the history of this period is to be accurately recorded.

It is said about Brown that, firstly, he was reluctant to resign, and secondly, according to Peter Mandelson, that it was Nick Clegg who finished Brown's political career by insisting that he go as part of any coalition deal between the two parties. Neither claim is right. Brown was, of course, desperate to keep Labour in power, and David Cameron out of office. But he knew – and had known for a long time – that he would have to make the ultimate sacrifice. It was Brown who told Clegg that he was willing to fall on his sword to bring about a historic realignment of British politics, and not the other way around. Once he had secured the passage of an electoral reform bill, and thus his own place in history, Brown told Clegg he would depart the stage.

Brown and Clegg had their first telephone call on the Friday evening. In it Brown made clear to Clegg that he was committed to seizing the historic opportunity to build a progressive alliance. Do not, Brown said, "doubt our political will". Citing their "common cause on Europe", Brown said that "ideologically, there are no big differences between us" and where policy differences did exist, on ID cards for example, Brown reassured Clegg that these could easily be dealt with. Brown also reiterated his commitment to electoral reform. Clegg gratefully told Brown: "I think our two parties working together are much more likely to achieve real change than anything we can do with the Conservatives."

Policy differences were never going to be the issue. During this call Brown then said: "There is something I need to speak to you about but I can only do it face to face." Clegg was in no doubt what he meant: Brown accepted he was a stumbling block to any deal and he would have to go. The two leaders did not meet until late on Sunday morning after Brown had returned from Scotland. At this stage the Liberal Democrats and Conservative negotiating teams had made significant progress in their talks, and Cameron was sensing victory. To avoid the media glare they arranged to meet in the office of Sir Peter Ricketts, then permanent secretary at the Foreign Office. It was at this private meeting between the two leaders that Brown told Clegg he would resign in the autumn once he had steered a bill on electoral reform through the Commons.

Brown's decision to offer his own head on a plate stunned Clegg and made him realise, for the first time, that Brown was serious about trying to make a Lib-Lab pact work. Until then the Liberal Democrats, partly because of electoral arithmetic – Labour plus Liberal MPs did not muster a parliamentary majority – and also because of real concerns that they would be damaged if seen to be propping up a losing Brown premiership, had not taken the option of working with Labour seriously.

Brown's offer to resign transformed the dynamics of the negotiations. Paddy Ashdown began to believe that his life-long goal of healing the fracture between the two parties might be possible. Then, on Sunday evening, Brown and Clegg met again but this time they were joined by their lieutenants, Peter Mandelson and Danny Alexander. When Brown's own position was discussed at this meeting he prevaricated. He refused to be as explicit as he had been with Clegg in private earlier in the day. Why? Not because he had changed his mind, but because he worried that the others in the room would leak the news. This apparent shift in position worried Clegg, who wondered whether Brown meant what he said. On the Monday at 5pm Brown decided to make public his promise to Clegg, and resigned as Labour leader. His critics claimed it represented the last throw of the dice from a Machiavellian politician. In truth he had always planned to go. The effect was electric. As the Lib Dems announced they were opening formal talks with Labour the Conservatives slammed one of their big cards on the table: the offer of a referendum on the Alternative Vote system.

For Brown, Andrew Adonis was the key figure in Labour's negotiating team. He spoke to him constantly. To show how serious they were about political reform, Adonis, at the behest of Brown, let it be known that Labour was open to the idea of holding a multi-question referendum containing not only an option for AV (which Labour would support) but also the Holy Grail for the Lib Dems: proportional representation. Despite such a sweetener it became clear on the Monday and Tuesday that the Lib Dems were cooling on the idea of a deal with Labour.

The Brown team was taken aback when the Lib Dems revealed that they had shifted their position on the deficit from that in their manifesto and were now calling for a more rapid fiscal consolidation. What explains this volte face? Incredibly the Labour camp received intelligence from Vince Cable, the Lib Dems' Treasury spokesman, that his party had been personally lobbied by the Governor of the Bank of England, Mervyn King, to adopt a much tougher position on spending cuts to placate the financial markets. When Brown challenged King on this directly later that evening, however, the Governor wouldn't be drawn.

By lunchtime on the Tuesday Brown had concluded that the talks were going nowhere. He began to prepare to go to the Palace. He stalled because Clegg called him and pleaded with him not to go. Clegg insisted that "I still think we can do a deal". Brown said Clegg must break off talks with the Tories to prove he was serious but Clegg evaded and kept demanding more time. In their third call at just before 7pm Brown said, "your time is up". At 7.20pm Brown walked out on to Downing Street with his wife, Sarah, and his two sons. It was the boys' idea to accompany their father: they had got used to watching history made outside No 10 and they now wanted to be a part of it themselves.

'Brown at 10', by Anthony Seldon and Guy Lodge, will be published in the autumn