The job cuts, also challenged by the Labour-led Scottish executive yesterday, were a centrepiece of the chancellor's plans to save £21bn a year and pre-empt Tory plans to promise "a thinner state".
With unions starting to lobby Labour MPs today to stop what they described as the "jobs carnage", Mr Brown said he would go ahead even if the unions were to strike.
He said investment and IT developments had made it possible to make jobs redundant, and pointed out that there were 600,000 vacancies in the labour market.
But the Scottish first minister, Jack McConnell, denied the claim that 20,000 civil service jobs would go in the devolved administrations.
He said his executive was doing its own review and would not agree to a figure set in England.
The Treasury has also had to concede that 13,000 of the 80,000 job cuts in England actually represent relocation.
Mr Brown said: "The civil service unions should be in absolutely no doubt we are going ahead with these reductions, even if they threaten to strike. That would be a mistake and counter-productive. We are going ahead with the reductions, they are both necessary and going to happen."
The cuts in "back office" jobs and efficiency savings, mainly through greater e-government, were projected to save £21.5bn a year.
John McDonnell, the chairman of the Public and Commercial Services (PCS) union parliamentary group, said in a statement it was seeking a meeting with the chancellor.
Sir Hayden Phillips, the permanent secretary at the Department for Constitutional Affairs, said he might have to rein back legal aid still further to meet the government's efficiency target. He said the department had already saved £136m in a year.
"We have got to have a fundamental look at legal aid in the criminal justice area," said Sir Hayden. "I think we may make savings there."
The Gershon review on efficiency in the public services, published on Monday, made no mention of cutting back services.
But Sir Hayden admitted that "more careful targeting of legal aid" could help achieve the aim of saving £290m over three years set out in the spending review.
The Liberal Democrats projected council tax would rise by up to 7% a year, and the shadow chancellor, Oliver Letwin, said a third-term Labour government would be forced to put up taxes.
He won support from Derek Scott, Tony Blair's former economic adviser, who said he would not "bet my house" on tax rises being avoided.
Keith Wylie, a PCS national officer, said he had received hundreds of calls from concerned public-sector workers. "This level of assault on the civil service is unprecedented and as a consequence staff morale has taken a hammering," he said.
"Staff do not know their future and the lobby will give them the chance to voice their opposition to this government's cruel attack on the valuable work that they do every day."
