At a meeting with Mr Blair and John Prescott, the party's deputy leader, the unions promised not only to support the unprecedented process - despite their initial reservations - but also to participate actively by staging debates and, in some cases, ballots of members on the manifesto proposals.
Labour strategists regard the agreement at the meeting with the general secretaries of the eight biggest union affiliates as a significant turning point - and one which contrasts with the hostility of many unions to Mr Blair's campaign to replace Clause IV of the party's constitution last year.
Then, only the Union of Communications Workers held a ballot of members, and the new union leaders' stance is said by Labour sources to indicate that they are anxious to be part of a process which most observers had suggested would diminish their influence over the content of the manifesto.
The unions' backing came as Tory ministers stepped up their a campaign to exploit Labour "confusion" on tax, after Clare Short, the party's transport spokesman, suggested that those earning between pounds 30,000 and pounds 40,000 should be paying more - a proposal rebuffed swiftly by the leadership.
Gillian Shephard, the Secretary of State for Education and Employment, claimed that Labour could be contemplating higher taxes for everyone earning more than the national average wage of pounds 20,600.
Brian Mawhinney, the Tory chairman, claimed that three Shadow Cabinet members - deputy leader John Prescott, Ms Short and education spokesman David Blunkett - had admitted to plans for higher taxes.
Meanwhile, Mr Blair will increase Labour's overtures to businessmen today by underlining his commitment to consult with business and industry before introducing a statutory minium wage.
Mr Blair will claim that his party is closer to business than at any time in its history, while the traditional Tory ties with business are loosening.Reuse content