The high profile campaign against the scrapping of Section 28 says it has evidence which will "blow a hole" in the Scottish Executive's plans for repeal.

The Scottish School Boards Association is expected to produce a government document from 1988 which says that Section 28 does not prevent teachers from discussing homosexuality.

A spokesman for transport millionaire Brian Souter, who has pledged money to the SSBA to help fund a campaign against the scrapping, has predicted this document will blow holes in the executive's justification for scrapping Section 28.

The government guidance produced by the then Tory Scottish Office said that Section 28 does not affect the activities of teachers or prevent the objective discussion of homosexuality in the classroom or the counselling of pupils about their sexuality.

The SSBA will claim that scrapping the law will remove a vital safeguard for children because of political correctness.

The executive's proposals have proved highly controversial pitting Scotland's leading Catholic, Cardinal Thomas Winning, against the Moderator of the General Assembly of the Church of Scotland, the Reverend John Cairns.

Cardinal Winning said homosexuality is perverse, Rev Cairns said this was unhelpful and children should be taught about life as it is.

Meanwhile, the Scottish Executive has said there is widespread public support for changing the law on the way teachers deal with homosexuality.

Ministers, who had invited comments from interested groups and members of the public, said 75% of the responses backed the repeal of Section 28, which bans the promotion of homosexuality in schools.

The executive is unanimously in favour of changing the law.