Ed Miliband has promised to be "ruthless" about pursuing Labour's public spending priorities, despite accepting the need for further cuts beyond the 2015 General Election.
The Labour leader said he would not make any promises on changes to the spending plans set out by the Chancellor George Osborne unless he can be "absolutely crystal clear" where the money would come from as he set out the "hard reality" facing the party.
He ruled out more borrowing to fund day-to-day spending, meaning any changes to the Chancellor's announcements in his 2015/16 spending review would require cuts from elsewhere or tax increases.
Mr Miliband said he and Shadow Chancellor Ed Balls were clear about the approach, and insisted the rest of the Labour Party should get behind it.
He told his party's National Policy Forum in Birmingham:
Nobody here should be under any illusions: the next Labour Government will have to plan in 2015 for falling departmental spending.
And our starting point for 2015/16 is that we won't be able to reverse the cuts in day to day, current spending unless it is fully funded from savings elsewhere or extra revenue, not from more borrowing.
So when George Osborne stands up next week and announces his cuts in day to day spending, we won't be able to promise now to reverse them because we've got to be absolutely crystal clear about where the money is coming from.
But Conservative Party Chairman Grant Shapps said Mr Miliband had "refused" to admit that Labour spent and borrowed too much while the party was in power.
Mr Shapps suggested the Labour leader's request for more spending and borrowing now "shows that he's too weak to stick to his promise not to spend and borrow more".
"Ed Miliband only offers more spending, more borrowing and more debt - the same old Labour approach that got us into this mess in the first place. Hard working people would pay the price with higher interest rates and higher bills", Mr Shapps added.