Gordon Brown has said he will not allow strike threats to derail his plans to axe 104,000 civil service jobs.
Brown admitted it was a "painful process"
The Public and Commercial Services Union (PCS) says the cuts will cause
"carnage" and warns it cannot rule out industrial action as a result.
But defending his spending review, the chancellor urged those talking of strikes to "think again".
"We will not be diverted from what's got to be done to get more resources to the front line," he told BBC Breakfast.
Opposition parties have been poring over the fine print of the spending review, which on Monday set out government spending plans for the next three years.
The Liberal Democrats argue the proposals assume that council tax will increase by an average of 7% every year.
The Tories say Mr Brown cannot deliver on his promises to make efficiency savings.
They say in the last recorded year, only 14,000 of the 88,000 extra jobs in education were teachers or classroom assistants.
But Mr Brown accused his opponents of falsely labelling front line staff as bureaucrats.
He dismissed suggestions his efficiency plans meant he had been until now been wasting public money.
Saying new technology now made the changes possible, he said: "It is not the case we could have done it before. We had to put the investment in first."
The spending review comes amid reports of tensions between No 10 and the Treasury.
But Mr Brown said he and the prime minister often laughed at such claims.
Insisting he was happy in his current job, he told BBC Radio 4's Today programme: "I'm really not interested in position, I'm interested in getting on with the job."
The job losses comprise 84,150 Whitehall jobs plus 20,000 from English councils and the devolved Scotland, Wales and Northern Ireland authorities. A further 20,000 Whitehall jobs are to be relocated from London.
MAIN PROPOSED JOB CUTS
Other losers include:
Office of the Deputy Prime Minister: 400
Foreign Office: 310
UK Trade and Investment: 200
International development: 170
Cabinet Office: 150
Northern Ireland: 130
Law Officers Dept: 50
Culture, Media and Sport: 30
The plans have provoked concern among some Labour MPs.
John McDonnell, chairman of the PCS parliamentary group, said they wanted to meet Mr Brown to discuss the implications.
The Labour MP for Hayes and Harlington added: "A hundred thousand job cuts
is not to be taken lightly, and Parliamentary colleagues have grave concerns for
their own constituencies as these measures begin to impact upon local services
and employment rates."
PCS general secretary Mark Serwotka said: "When this government was elected
they imposed the windfall tax to pay for initiatives like the New Deal.
are using their own workforce to pay for their policies.
"We cannot rule out industrial action in the face of such a serious
TUC general secretary Brendan Barber said: "These cuts cannot be made without hitting the quality of public services.
"They will deal civil service morale a bitter blow just as staff support is
needed for change."
Shadow chancellor Oliver Letwin said Labour would be forced to put up taxes if re-elected, to
pay for the increased spending.
He told Today programme: "If we want to get money on to the
front line of our public services, if we want to cut down on the flab in
Government, it's no good the chancellor saying that he is going to do more and
more, and run more and more of people's lives.
"He has actually got to have a
change of lifestyle.
Liberal Democrat Treasury spokesman Vince Cable said Gordon Brown and Michael Howard appeared to be playing a "macho game of brinkmanship" in sacking public servants.
And he said Mr Brown had not explained how he would make £15bn of the cuts.