Almost 100,000 workers took part in strike action over pay in April

The Public and Commercial Services Union also attacked the "disgraceful" way Chancellor Gordon Brown announced the cuts in his last Budget.

It said it would use every means it could to oppose the job cuts and plans to move staff away from the South East.

The Treasury has rejected the union's criticism, saying cuts made in the centre meant more frontline staff.

Members at the PCS annual conference voted for a national demonstration and approved a campaign of industrial action.

'Sacked by text'

PCS leader Mark Serwotka said the way ministers had cheered the televised announcement of job cuts by Mr Brown was "disgraceful" and "obscene".

It was the "biggest attack" made on the civil service, he said.

BBC labour affairs correspondent Stephen Cape said Mr Serwotka had compared the government's action to telling people they were to be sacked by text message.

Delegates at the Brighton conference voted unanimously to lobby MPs and even to seek a judicial review to prevent the cuts.

If the government wants to reduce the number of civil servants they need to explain what services they are prepared to dispense with PCS conference motion

They also expressed fears about plans to transfer up to 20,000 jobs in London and the South East to other regions.

London PCS delegate Phil Pardoe said the capital faced a relocation of workers on a "massive scale" which would have a negative impact on jobs and services.

He warned redeployment could also reduce ethnic diversity within the civil service because a high proportion of its ethnic minority workers came from London.

The conference started by telling of civil servants' "anger and concern" over the proposed job losses.

'Party politics'

Its opening motion said: "It is clear that the announcement and the singling out of the civil service for attack was more to do with party politics and the 2005 general election.

"If the government wants to reduce the number of civil servants they need to explain what services they are prepared to dispense with."

The government wants to cut 30,000 jobs in the Department for Work and Pensions and 1,500 at the Department for Education and Skills (DfES).

A further 10,500 would be shed by merging the Inland Revenue and Customs and Excise.

We are making savings at the centre, so that we can increase investment at the front line and employ more doctors and nurses, more teachers and police Paul Boateng, chief secretary to the Treasury

Tony Conway, of the union's Midlands branch, said senior managers in the DfES doubted the cuts could be achieved.

He claimed the department was in a state of "panic", with staff fearing for their jobs.

But the chief secretary to the Treasury, Paul Boateng, defended the government's actions.

He said: "Mark Serwotka is wrong. We are making savings at the centre, so that we can increase investment at the front line and employ more doctors and nurses, more teachers and police."

The PCS, which has more than 300,000 members, is already involved in a pay dispute with the Department for Work and Pensions.

On Monday, staff at job centres and benefits offices voted for two days of strike action in July on the issue.

It follows widespread disruption in April when almost 100,000 civil servants took part in a 48-hour strike over low pay.