Alistair Darling has promised to begin spelling out the "hard choices" Labour will make on future public spending as early as this autumn.
The Chancellor said all the parties would have to detail their plans for re-balancing the country's books before the next general election.
"You have to set out those priorities," he said.
"Apart from anything else, the next election could see a government elected until 2015 - it will need a mandate for the hard things it's got to do as well as the things it wants to do to improve the country.
"Politics is about choices, and people when they go to the polls - whenever it is - will want to know where all three political parties stand."
His comments come after weeks of rowing between Labour and the Tories about their respective plans for public spending beyond the next election, which is expected early next year.
The Institute of Fiscal Studies has predicted that whoever is in power will have to introduce significant cuts in order to bring down massive national debt.
Mr Darling said the current economic uncertainty meant it was impossible to "settle everybody's budget down to the last tray of paperclips between now and 2013".
But he added: "I did say we would need to come back to this at the Pre-Budget Report because ... we are committed to the finances in this country being sustainable."
Mr Darling was emphatic that VAT would be returned to 17.5% at the end of this year, despite recent speculation that he might review the proposed rise.
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