David Davis, the shadow Deputy Prime Minister, is to vote for the retention of Section 28 today if the Tories' alternative plans for school sex education fail in the Commons.
Mr Davis is likely to back an amendment by Ann Widdecombe, a former shadow home secretary, when she launches a last-ditch attempt to retain the clause. Up to 100 Tory MPs may support Miss Widdecombe and Edward Leigh, a former minister, in a vote during the report stage of the Local Government Bill.
Section 28 of the 1988 Local Government Act, which bans the "promotion" of homosexuality in schools, looked doomed after a cross-party group of MPs voted for its abolition in the committee stage of the Bill.
As a compromise, Mr Davis has drafted an amendment replacing the clause with plans to give parents a right to a ballot on the materials used in their children's sex education.
If, as expected, the amendment is crushed by Labour's majority, he will vote for the Widdecombe/Leigh amendment. "If you get rid of Section 28 without replacing it with any safeguards then it is not worth getting rid of at all," a senior Tory source said.
Divisions will be highlighted by the free vote, with those opposing Miss Widdecombe led by John Bercow, a former work and pensions minister. Others among the 40 or so Tory MPs expected to join Mr Bercow are Kenneth Clarke, Michael Portillo and David Curry.
Mr Bercow quit his post last year over Tory opposition to gay adoption, triggering a "unite or die" ultimatum from Iain Duncan Smith to MPs upset at his handling of the affair.