Granita, a byword for the pact that has hung over New Labour for a decade

The Blair-Brown deal Single typewritten sheet appears to provide first evidence of how Brown secured guarantees on his social agenda

The dinner date between two front benchers was destined to become the stuff of Whitehall legend: Tony Blair, Gordon Brown and an after hours tête-à-tête at Granita on Upper Street, Islington on 31 May 1994.

They chose the fashionable restaurant, popular among left-leaning aspirants, not knowing it would later become a byword for a power-sharing pact which would have a lasting impact on the future government.

Theirs was so significant a dinner that it has been revisited by political analysts as the defining moment for the bid for power by New Labour. Its sense of intrigue is still so immense that it is soon to be the subject of a Channel Four docu-drama, The Deal, starring Michael Sheen as the bright young premier.

It was there, sitting on awkward steel chairs and snacking on lentils and corn-fed chicken, that the Prime Minister and the Chancellor forged the agreement which led Mr Brown to announce the following day that he would not be standing for the Labour leadership. The shadow Chancellor, a former politics lecturer and television current affairs editor, announced that he would not be a candidate for Labour's vacant leadership, but would "encourage" his friend, the shadow Home Secretary, to run.

Mr Brown's grand gesture was universally admired and it left the media perplexed. In a decade which seemed to be driven by self-aggrandisement and Thatcherite individualism, he was viewed as a loyal friend who was prepared to sacrifice ambition for the sake of a greater political purpose.

But the move was not altogether altruistic and had some hard, calculated planning behind it.

As John Rentoul wrote in his biography of Mr Blair, Mr Brown's position was the stronger in the formative years of their friendship.

After the 1992 election, Mr Brown had to discourage speculation that he might run against John Smith for the Labour leadership. But just two years later, Mr Blair and Mr Brown's 11-year friendship reached its most testing moment, when support for Mr Blair turned out to be overwhelming.

They first met in 1983, as Labour's two youngest MPs. For the first nine years, Mr Brown was something of a guru to Mr Blair. Friends attest to Mr Blair's awe of Mr Brown and say he was enormously dazzled by Mr Brown's power.

But by the night of the Granita dinner, it had become clear that the shadow Chancellor - for years regarded as the senior partner of the Brown-Blair partnership - had been outstripped by his friend and rival.

The note prepared by Peter Mandelson for press briefing on the day after their fateful encounter reveals their assessment that each could rely on similar levels of support in the Labour Party.

But they privately knew that the young barrister could reach out to "Middle England" in a way Mr Brown could not. As the Mandelson note said: "Two opinion polls suggest that Tony has the edge."

In return for ceding the crown, Mr Brown signalled his intention to be the power behind the throne. In wresting an apparent "guarantee" from Mr Blair that a Labour administration would be driven by a Brownite "fairness agenda" he was setting the course that would still be driving New Labour more than nine years later.

In Donald MacIntyre's biography of Mr Mandelson, he argues that the word "guaranteed" was one that Mr Blair's camp have sought to minimise and control while Mr Brown's supporters have attempted to exploit and maximise. There were said to be furious arguments, shouting and rowing over concessions and demands. A Blair aide was believed to have said that the Chancellor had "psychological flaws" and some in Mr Brown's camp were removed from the inner sanctum of government.

Mr Blair showed no sign of stepping down for Mr Brown before the 2001 election, a course of action which is rumoured to have been one of the promises made over that furtive dinner.