Academies run by a high-profile sponsor are asking staff to sign contracts giving them tougher conditions, including halving the amount of paid maternity leave.
The National Union of Teachers in Southwark, south London, is urging members not to sign contracts at academies sponsored by the charity Ark (Absolute Return for Kids). It has also criticised contracts introduced at academies sponsored by Carpetright entrepreneur Lord Harris of Peckham.
Southwark is becoming increasingly dominated by academies. By 2009, there will be eight in the borough out of a total 16 secondaries. There is already only one community school.
A comparison of Ark academy policy and the standard school contracts, shown to the NUT, suggests the length of maternity leave on full pay has been more than halved from nine to four weeks. Teachers are subject to a six-month probationary period and written warnings remain live on file for twice as long. They are required to work around five extra days a year.
Michael Davern, NUT joint secretary for Southwark, said Ark was using "aggressive management techniques".
Ark is a charity founded by Arpad Busson, the French multi-millionaire, who is also the father of model Elle Macpherson's two children. It has opened three academies so far: Burlington Danes in Hammersmith, King Soloman Academy in Westminster and Walworth Academy in Southwark.
It plans to open another in Southwark in 2008 and one in Brixton for 2009.
The Harris Federation of Schools is made up of six academies, including three in Southwark.
Last week Lord Harris offered staff a 20 per cent discount at his chain of carpet and flooring stores. This was raised from the original offer of 15 per cent to bring teachers into line with other Carpetright employees. Perks, including Marks and Spencer vouchers at Christmas, and bonuses for improved school performance, were also highlighted.
But the Southwark union said the Harris Federation had been uncooperative about teachers' pay and conditions and had not let them see a copy of their teachers' contract.
Mr Davern said: "We know there are many unhappy members of staff."
Kathy Duggan, an NASUWT representative in London, said although Ark had been co-operative with the unions before Burlington Danes academy opened in 2006, the majority of the original teachers had now left.
Teachers who transfer to an academy from its predecessor schools remain on existing local authority contracts. When new teachers join, they are offered the academy's own contracts.
Problems with teachers' contracts are expected to increase as the numbers of academies rise.
Lesley Smith, a spokeswomen for Ark said: "The contract terms are broadly similar to those that apply in community schools."
Dr Daniel Moynihan, chief executive of the Harris Federation, said they were co-operating fully with unions and most teachers had signed their new contracts.