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 unless he can be "absolutely crystal clear" where the money would come from as he set out the "hard reality" facing the party.
Stressing the need for discipline in bringing down the deficit, Mr Miliband 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.
The Labour leader 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.
"We will show the discipline the challenge of our times demand. It is the only way we can credibly change our country. It's a hard reality. I am clear about it. Ed Balls is clear about it. And everyone in the Labour Party should be clear about it too."
But he drew inspiration from the transformative post-war Labour prime minister Clement Attlee to say that even in tough times it was possible to make a difference and "give people hope".
Setting out how he would cope with his economic inheritance if he walked through the door of 10 Downing Street, Mr Miliband said: "If we win that election in 2015, we will need to be ruthless about our priorities in public spending. It is why we have also said we don't believe it is right to say that we should carry on paying the Winter Fuel Allowance to the very wealthiest pensioners, those with incomes over £43,000 a year. It is why we won't be able to make it a priority to restore child benefit to families who earn over £50,000 a year."