The national Union of Teachers has raised "serious doubts" about the independence of the statutory body that advises ministers on teachers' pay.
The School Teachers' Review Body advised the Government on pay rises for the next three years that prompted thousands of teachers to take strike action.
Bill Cockburn, the retiring chairman, sparked a public row when he told The TES in April that striking teachers would lose the "great respect" they had earned.
Christine Blower, acting general secretary of the NUT, was furious with the criticisms at the time.
She has now written to the STRB expressing her "extreme concern" over its handling of the 2006-2008 pay settlement.
In a letter disclosed to The TES, Ms Blower says the teaching unions have been promised a decision on a pay review for the past three months - but the decision has been repeatedly postponed. Teachers were awarded pay rises of 2 per cent in September 2006 and 2007, but with the proviso that it could be reconsidered if inflation were to increase significantly.
Retail price index inflation was 4.3 per cent in the past year, but neither the STRB nor the Government has yet agreed to reopen the pay deal as the proviso allows.
Ms Blower wrote of the union's "considerable concern" over the STRB's failure to respond to the request for a pay review.
"This delay is bound to raise questions from our members including serious perceived doubts about the independence of the STRB," she wrote.
A spokeswoman for the STRB would say only that the review body was finalising its response at the moment.
Other teaching unions, while concerned that teachers' pay rises have fallen behind spiralling inflation, are nonetheless happy with the STRB's neutrality.