Official 2002 figures show that 3,450 teachers benefit from "safeguarding" arrangements for perpetuity, granted by local authorities when they move to jobs on a lower salary after school reorganisations or closures. The NASUWT, the second-largest teachers' union, claimed the real figure was much lower.
In September, the unions, apart from the National Union of Teachers and Welsh union UCAC, signed a deal with the Government and employers that implied further talks but agreed the principle that pay protection should disappear after three years.
Now The TES understands the Association of Teachers and Lecturers and the NASUWT are attempting an 11th-hour reversal because of concerns about the unpopularity of the deal, which was an attempt to rationalise safeguarding arrangements nationally.
The deal was largely concerned with arrangements for changing pay for senior staff. But it was also intended to mop up pay protection deals that had been made up largely ad hoc by local authorities, retrospectively.
Chris Keates, NASUWT general secretary, denied making a U-turn and said it had always been agreed that the issue would be revisited.
But Andy Inett, from the employers, said: "The overarching principle we arrived at was that all safeguarding should be limited to three years.
"We are now discussing with our partner unions and the Government whether there could be an exception for those benefiting from existing safeguarding arrangements."
Another senior official from one of the two classroom unions said the unions had originally agreed the three-year limit as part of a very complicated agreement with a tight deadline.
But there were now concerns about the ammunition it would provide to the rival National Union of Teachers.
The damage-limitation exercise comes as the "social partnership" between unions, Government and employers is already under strain. The National Association of Head Teachers has walked out over the workforce agreement, and those still in the partnership have until Wednesday to agree a new pay rise for teachers.
The current multi-year package, which meant a 2.5 per cent rise last month, topped up to 3.25 per cent in September, was pegged to projected inflation levels and was condemned by both the ATL and NASUWT when it was announced by the Government in 2003.
This time round they will have to put their names to its successor, to run from September 2006 to August 2008, or face the disintegration of the close working relationship they have established with the Government.
Education Secretary Ruth Kelly, in a letter to the School Teachers' Review Body, has already stressed the need for an affordable deal that takes account of low inflation and the need for education funding to meet other priorities.
The cost of implementing the final phase of the school workforce agreement also makes it unlikely that a deal can be achieved that is significantly better than the one the unions opposed so fiercely.