Shadow Cabinet sources say Tony Blair intends to fight to win the vote against a motion from the Transport and General Workers' Union, which demands a manifesto pledge from Labour to a minimum wage of pounds 4.15 an hour.
The motion is likely to be supported by Unison, the public sector union, and the GMB general union. Between them, the three unions hold one-third of the conference vote. But Mr Blair, who told the TGWU conference on Monday that the unions no longer "have an armlock" on Labour, will try to persuade delegates rather than announce in advance that he will ignore the vote.
A spokesman for Mr Blair said: "The argument has been laid out very clearly and will be laid out very clearly, and we think it will be won." Motions from local parties to Labour conference, submitted last week but not yet published, are believed to be divided equally between support of and opposition to the leadership's policy.
Mr Blair is expected to tell union leaders that they do not have a mandate for opposing him. On Tuesday the TGWU conference passed a resolution supporting the formula for a pounds 4.15 minimum wage, but it does not demand a manifesto commitment, calling only for the campaign for a minimum wage "to be a key priority in the run-up to the next election".
The GMB at its conference called only for a minimum wage to be "set at a realistic level", in line with the Labour leadership's policy to set up a Low Pay Commission, including employers, to decide the level after the general election.
The leadership will be backed by several smaller unions, including the Communication Workers' Union, which represents low-paid postal workers. Dave Ward of the union said yesterday: "To impose an artificial target on the party is stupid - it won't do the unions any favours."
Allies of Mr Blair have rejected the "Old Labour" option of briefing journalists that he will ignore the vote if it goes against him.
One said: "That is the old politics - he is trying to work out what is the best thing to do and win the argument."