The debate over spending between the party leaders is expected to intensify
The Tories have rejected claims that they are planning "massive" cuts after the next election as the party leaders clashed over their spending plans.
Gordon Brown said the Tories had revealed plans to reduce expenditure in many areas by 10% in the three years after 2011, hitting basic services.
David Cameron said Labour would be forced into big cuts of their own due to their "appalling" economic record.
Independent experts believe cuts will happen whoever wins the next election.
But the BBC's Political Editor Nick Robinson said neither party was comfortable talking about where any spending cuts, which he said were inevitable, would fall.
The party leaders exchanged barbs over their spending plans in the Commons, with Mr Brown seizing on comments made earlier by Tory health spokesman Andrew Lansley about the party's spending plans.
In a BBC interview, he appeared to suggest that, in order to protect "priority" spending on the NHS and schools, a future Conservative government would cut expenditure in other areas by a total of 10% between 2011-5.
He was asked how the Tories would fund their pledge to maintain real-term increases in health and international aid spending - given the party's commitment to reduce what it says are Labour's unsustainable debt levels.
This is the day when the Conservatives have revealed their true manifesto for this country
"Unfortunately what this means is that there is going to be very powerful spending constraints elsewhere across government," he said.
He continued: "That does mean over three years after 2011 a 10% reduction in departmental expenditure limits for other departments. It is a very tough spending requirement indeed."
In response, at prime minister's questions, Gordon Brown said such a policy would result in "massive" cuts to "vital" services.
In contrast, he said under Labour spending would rise, in real terms, in each of the next five years.
"This is the day when the Conservatives have revealed their true manifesto for this country," he said.
"The choice is between a government prepared to invest in the future and a Tory Party which is going to cut."
Mr Cameron said the Treasury's own projections, set out in this year's Budget, would result in a sharp fall in spending after 2011 - amounting to 7% for some departments.
He said the government's economic legacy was catastrophic, with the "biggest budget deficit in the country's history".
The next election would not be about investment versus cuts, he said, but Labour's record over the past 12 years.
"It will be about the mismanagement of the public finances, the appalling deficits he has left and and his plan for cuts," Mr Cameron said.
Mr Lansley later sought to clarify his remarks, saying the two parties were facing the same pressures on spending.
"The issue is how do you deliver more for less in government," he said.
In a statement, the party said Mr Lansley was pointing out that Labour was trying to "deceive" the public by suggesting it could avoid spending cuts after 2011.
Mr Brown was misleading the public when he said spending would continue to rise as he was ignoring the costs of paying interest on the spiralling public debt.
But Health Secretary Andy Burnham said the Conservatives had "let the cat out of the bag" and revealed the difference between the two parties.
He added: "We have said that we believe the right thing for the country is to invest now, to help it through difficult times. The Tories have said they'd do something different - that they would have to cut this year, and they they would cut next year."
Shadow Chancellor George Osborne said the government's claims were "dishonest" and called for a "grown-up discussion" about how to protect front-line services.
"Whether Gordon Brown is honest enough to be part of that discussion remains to be seen," Mr Osborne added.
Chancellor Alistair Darling announced plans in the Budget to borrow a further £500bn over the next four years.
The debate over public spending is set to intensify in the run-up to the election with both main parties having to explain how they will reduce the spiralling level of public debt without being forced into both tax rises and deep spending cuts.
If this is a sign of things to come in the debate on public spending then we're in for a pretty miserable year
At this stage, neither party have spelled out their detailed spending plans beyond 2011.
In its Budget analysis in April, the respected Institute of Financial Studies said the country faced "two parliaments of pain" as the next government had to reduce debt levels and bring the public finances under control after the damage caused by the banking crisis and recession.
It said there was £90bn "black hole" in the public finances and it would cost £2,480 in higher taxes or spending cuts per family to bring the budget back into balance.
Labour disputed this figure and said the IFS had not understood the full picture. BBC Economics Editor Stephanie Flanders said that "the government's own numbers imply a 10% real cut in spending on other departments between 2011 and 2013 if the NHS and DFID are protected".
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