Proposals to relocate the Qualifications and Curriculum Authority from its Mayfair home to an unspecified location outside London from 2008 are a "disaster waiting to happen" for schools, it is claimed.
The Public and Commercial Services Union says up to 95 per cent of staff could leave the QCA when it moves - just as it is attempting to implement the most complex curriculum reforms in its history.
Changes scheduled for 2008-10 include the introduction of specialised diplomas, the launch of new functional skills exams, a new statutory key stage 3 ICT test and the removal of coursework from many GCSEs.
Critics have dubbed 2008 the "meltdown" year for curriculum reform. The plans to move are part of budget cuts which will reduce the QCA's funding by at least 20 per cent from the pound;150 million to pound;120m by 2008.
Its workforce is being cut by a quarter this year.
The regulator is being forced to move following the Lyons review of spending across Whitehall, instigated by Gordon Brown, the Chancellor, in which thousands of civil servants' jobs are moving to the regions.
Dean Rogers, London and south-east regional officer for the PCS union, which represents hundreds of QCA staff, said experience suggested most would resign rather than leave London.
He said: "The most optimistic prediction suggests 85 per cent of staff will leave. But we fear, from meetings with staff, that it could be as low as 5 per cent choosing to relocate.
"These are not back-room staff. They are experts responsible for developing a 21st-century curriculum. They are working on a set of highly complex developments which will culminate in 2008-10."
A QCA spokesman said: "Any relocation... would be carefully planned to ensure that QCA's programme of work could continue."
The Department for Education and Skills refused to comment.