Unions vehemently opposed to academies have signed a pay and conditions agreement with the largest sponsor of the new schools, guaranteeing teachers a day a week for marking and preparation.
The National Union of Teachers' executive will face questions from delegates at its conference in Gateshead this weekend after joining six other unions in signing a deal with the United Learning Trust.
Nick Grant, secretary of the NUT's Ealing branch, which will call for a tougher stance on academies at the conference, said he was concerned that such agreements might compromise the union's ability to speak out.
The trust is an offshoot of the Church of England, which is sponsoring three academies already in existence and is planning another six. The new, largely state-funded independent schools are not governed by national pay and conditions arrangements.
Last week, Steve Sinnott, NUT general secretary, described the academies programme as "immoral" and said the union would oppose each new school.
The academies policy is among the most contentious in education. MPs last week called on the Government to stop building the new schools until they had been properly evaluated.
Under the terms of the agreement, drawn up after two years of talks, the trust has agreed to recognise the NUT, the five other teachers' unions and the support staff association Unison.
Teachers have been guaranteed salary rates which unions and the trust said were slightly higher than those for mainstream schools.
The agreement stipulates a 37.5-hour working week, slightly more than that laid down for teachers in other schools.
But it also guarantees 20 per cent of that time for marking and preparation - double the amount that teachers across England and Wales are due to have from September.
Barry Fawcett, NUT assistant secretary, said: "As with grant- maintained schools, we were opposed to them but still had to seek to safeguard our members' interests in them. That is what we are doing here."
Joe Boone, assistant secretary for industrial relations at the National Association of Schoolmasters Union of Women Teachers, which has also opposed academies, said the union had got a "reasonable" deal out of the trust, which was ahead of other sponsors in working with the unions.
"If there were several different organisations looking to run an academy, I know which one I would be favouring: the United Learning Trust," he said.
Ealing union branch has tabled an amendment to an NUT conference motion on privatisation urging the union not to talk to prospective academy sponsors.
It is expected to be opposed by the union's executive.
Nick Grant said: "I don't see a problem with defending the rights of our members who work in academies. But we have to be careful, in entering into talks, that we do not compromise our fundamental opposition to academies."
The National Secular Society said it also had concerns about the agreement, as it does not stop ULT schools from employing Anglican teachers in preference to others, as is possible in mainstream Church of England schools.
Charlotte Rendle-Short, ULT deputy chief executive, said: "The agreement is very good news for staff within our academies."
There are 17 academies so far. Ministers have pledged to have 200 in development by 2010.
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