Former education secretary Estelle Morris may vote against government plans to reform schools, reports say.
Lady Morris quit as education secretary in 2002
Tony Blair is facing a Labour backbench rebellion over a proposal to create "independent" state schools in England.
Lady Morris said the plan was "at best a distraction, at worst a change of direction", the Guardian reported.
David Blunkett, another ex-education secretary, is expected to work behind the scenes as a "bridge" between the government and MPs, sources say.
Under the government's education white paper, some schools would become self-governing trusts, with more freedom from local authorities over their finances, staff and which pupils they take.
Lady Morris, who quit as Education Secretary in 2002 and stood down as MP for Birmingham Yardley at the last election, threatened her first vote against party policy over the school reforms.
Conservative leader David Cameron has offered provisional support for the bill, to be published early next year.
But some Labour MPs see it as the beginning of a return to selection in secondary education.
Lady Morris said: "Show me a school which has changed its admission policy to attract more children from poor backgrounds with uncooperative parents.
"When schools change their admissions policy it is to attract more able children or a better balanced intake."
Mr Blunkett, who resigned as Work and Pensions Secretary last month, is not thought to be fully behind the white paper plans.
But Westminster sources say he is expected to use his new status as a senior backbencher to attempt to help a compromise between MPs and ministers.