Between 700 and 800 jobs are to be lost at the Department for Education and Skills as part of efficiency cut-backs.
Unions claim the threat of redundancy is already damaging morale in the department and may hamper work with schools.
The DfES had previously dismissed reports that up to 1,000 staff would go as speculation.
But this week it confirmed cuts would be made at all its sites, London, Runcorn, Darlington, Sheffield, and its headquarters in Westminster.
The review anticipates a Government-wide efficiency drive by the Treasury, which reports in the spring.
The DfES said it was unable to say if any redundancies would be compulsory.
But the Public and Commercial Services union expects staff to be offered voluntary redundancy and early retirement.
Alex Flynn, spokesman, said the cuts would still be a great blow to staff, particularly in high unemployment areas such as Runcorn. He also said there were concerns about the impact of the cuts on support for schools.
"Staff morale has taken a real battering," he said. "There is a great deal of uncertainty. "We need to know whether this review is about making the DfES function better or simply an excuse to cut jobs."
The FDA association for senior civil servants, does not expect any of its members to be made compulsorily redundant.
But John Merson, FDA national officer, said that the union was concerned:
"We will be taking steps to protect our members and the service they provide for the country," he said.
Members of the PCS in departments including the Home Office and Department of Work and Pensions were today due to join a second day of strikes over pay.
Staff are also considering action at the Department of Health, where 38 per cent of staff in Whitehall are due to be cut this year. But the PCS said it was far too early to speculate about the possibility of industrial action in the DfES.