Fifteen thousand jobs could be lost at the Ministry of Justice, a union has warned - after a letter revealed plans to cut £2bn from its budget.
The Public and Commercial Services Union says 15,000 of the department's 80,000 jobs are at risk.
With £4bn of the department's £9bn budget fixed for legal aid and prison costs, the cuts could bring courts "to a standstill", the PCS said.
Finance chief Ann Beasley's letter says "efficiencies... will not be enough".
Ms Beasley, the director general of finance for the ministry, said in her letter to senior colleagues that "many savings will have to be made quickly, within the first 12-24 months".
She added: "Efficiencies alone will not be enough. It will mean we have to look at every opportunity to work differently and better. It will also mean there will have to be less of us."
Staff would be "understandably concerned" so, she writes, "it is important that you take every opportunity to talk to them, listen to their concerns, invite their ideas and views and be honest about what we do and don't know and when we will tell them more".
Whitehall departments, with the exception of the Department of Health and International Development, are being asked to outline proposals to reduce costs by between 25% and 40%.
Precise detail of where the cuts are to be made are expected to be hammered out with Chancellor George Osborne and his cuts "star chamber" during September ahead of the government's spending review due on 20 October.
The cuts are part of the coalition's plans to wipe out the UK's Budget deficit by the end of the current parliament.
The main areas of the Ministry of Justice's responsibilities are the courts system and prisons as well as other areas such as data protection and the Information Commissioner.
The PCS said the cuts would be the "equivalent to the entire budget for prisons, or the money the department spends each year on courts and tribunals".
It said the letter was the "first indication of the true scale of cuts" the coalition was imposing.
PCS general secretary Mark Serwotka said: "This is the first indication of the true scale of the cuts being imposed upon departments by this coalition government, and it paints a devastating picture.
"It is clear that the civil service will simply not be able to cope. We will take every opportunity to remind the government and the public that there is an alternative and these politically-motivated cuts are entirely avoidable."
A Ministry of Justice spokeswoman said: "The Ministry of Justice has submitted proposals for savings to HM Treasury.
"Like all departments we were asked to plan for savings of 25% and 40% as part of the coalition government's deficit reduction plan.
"We are discussing options with Treasury and will not provide a running commentary on the process. No decisions have been made. The outcome of the Spending Review will be announced on 20 October."
In the letter Ms Beasley says that after spending review there would be a departmental business plan which, she said, would "explain to the public how, over the next four years, we will use the money that has been allocated to us".