Labour rebels are warning that 50 MPs are preparing to vote against Tony Blair's schools reforms, which would leave the Government relying on Tory support to gain a Commons majority for the controversial plans.
Both Mr Blair and Ruth Kelly, the Education Secretary, are to launch a fresh attempt to win over wavering backbenchers ahead of the long-awaited vote on Wednesday.
But the scale of their task was underlined by a BBC survey, which calculated that at least 39 Labour MPs will defy Mr Blair in the vote on the Education Bill's second reading.
Although the size of the revolt appeared last night to be falling, a rebellion on that scale would still wipe out Labour's majority and leave Mr Blair with the political embarrassment of winning the day with Tory votes.
Ms Kelly hinted at further concessions yesterday as she promised to "continue dialogue with colleagues" over the Bill, which creates "trust schools" backed by private companies.