Some areas could see unemployment rise by 40%, the TUC argues
Public spending cuts would create a "double-quick, double-dip" recession and push unemployment over four million, the TUC's leader has warned.
Brendan Barber called it "astonishing" that demands for reducing the budget deficit were being seen as a priority, rather than funding economic revival.
Speaking ahead of the TUC Congress in Liverpool, Mr Barber said the outlook was "very precarious".
But Gordon Brown says the economy is "on the road to recovery".
The TUC's congress, which starts in Liverpool on Monday, will be the last one before the next general election and it comes at a time of strained relations between the unions and the government.
Mr Brown will deliver an upbeat message to delegates when he speaks on Tuesday, declaring "we are on the road to recovery", though he will say this will not be automatic and the recovery will need to be nurtured.
The prime minister will say that he will protect frontline jobs and urge TUC members not to disrupt the government's efforts with industrial action.
Mr Brown will also say: "People's livelihoods and homes and savings are still hanging in the balance, and so today I say to you: don't put the recovery at risk.
Brendan Barber says cuts would only promote recession
"We have to make tough choices in public spending and we will need the support of the labour movement in protecting the front line first."
The government says it wants to halve its budget deficit - expected to reach £175bn this year - within four years.
Unions are alarmed at suggestions that this could mean slashing some areas of spending, possibly resulting in large-scale redundancies.
Mr Barber, TUC general secretary since 2003, said: "Cut the stimulus off and the economy would go into decline again. Public spending cuts will provoke a double-quick, double-dip recession.
"Unemployment could well exceed four million and it would take many years before there was any chance of returning to anything like full employment. That would scar for life a whole generation of young people."
Meanwhile Liberal Democrat treasury spokesman Vince Cable called for the debate to move on, saying "big ticket areas" such as "defence commitments, public sector pensions and tax credits" needed to be examined.
BBC political correspondent Paul Rowley said any disagreement with the unions could be "highly damaging" to Labour's leadership ahead of the next election.
Mr Brown would spend his visit trying to "smooth things over" with his party's biggest political backers, he added.
The TUC has published a report analysing the effects of possible public spending cuts on the 25 local authorities with the highest levels of unemployment.
It suggests that areas such as Liverpool, Leicester and Middlesbrough would suffer increases in unemployment of around 40%.
The report also warns that a 10% cut in public sector staff would lead to 700,000 workers being laid off.
Mr Barber said deficits were an "inevitable symptom" of a recession, adding: "When the economy shrinks, governments need to spend more on benefits but get less tax income, and there has been a spectacular collapse in tax income in this recession."
He told BBC One's Politics Show: "Our biggest problems are the recession and unemployment. That's what the government needs to be focusing on.
"A time will come when we need to get the deficit down, but that's not the stage that we are at."
Public spending looks set to be one of the biggest issues in the lead-up to the next general election.
The Tories say the government is gathering too much debt, but Labour says the Conservatives would reduce funding for projects vital to restoring the economy's fortunes.
The prime minister will tell the TUC congress that public sector jobs would be at greater risk under the Conservatives.
Mr Barber said: "If a Conservative government was elected that was taking the axe to the public sector, that might well provoke a reaction. There would be that danger."
The TUC general secretary has met Tory leader David Cameron twice in recent weeks but he said would not disclose what had been discussed.
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