But the Scottish Executive is remaining firm and says it will not support Mr McMahon's amendment, which will be debated by the local government committee of the Scottish Parliament.

We have made our position clear on that. There is no question of any change

Scottish Executive spokeswoman

He wants to place the importance of marriage in legislation to replace Section 28 - the law which governs what can be taught about homosexuality in schools - while stressing that children should not suffer intolerance because of their family background.

Millionaire Scots businessman Brian Souter has offered to end his high-profile campaign against repeal if the amendment is accepted.

The Catholic Church, another vocal opponent of abolition, says it backs Mr McMahon.

Parents' concerns

A spokeswoman for the Scottish Executive said: "It will be a matter for the committee as to how it votes.

"The executive tabled its own amendment to the bill in the light of parents' concerns and it stands by that amendment.

Michael McMahon: Amendment

"We have made our position clear on that. There is no question of any change."

Executive sources said they saw little likelihood of Mr McMahon's amendment being passed by the committee on Tuesday.

The debate coincides with the outcome of anti-repeal campaign group Keep the Clause's privately-run ballot on the issue.

Ministers' refusal to budge on the issue comes as speculation mounts that some Labour MSPs will defy the party line and back Mr McMahon, who represents Hamilton North and Bellshill.

They are understood not to want to be seen to vote against marriage and up to 20 backbenchers are understood to be sympathetic to a rebellion.