Labour's spending plans remain a "big unknown" due to the lack of any clear timeframe on the pledge to erase the deficit, according to an influential think-tank.
The Institute for Fiscal Studies (IFS) said that the lack of clarity on when the books would be balanced meant the party could introduce "significant spending cuts or tax rises" early on, or none at all.
IFS Director Paul Johnson said: "They are committed to getting a current budget balance - a current budget surplus - by the end of the parliament. That's an entirely credible fiscal policy. But they continue to not to give us any sense as to how quickly they want to achieve that.
"That could involve significant spending cuts or tax rises over the next three years if they want to get there within the next three years, or it could actually involve no spending cuts in order to just achieve that by the end of the parliament."
Johnson added that all the major parties were "just making up numbers" on the amount they could raise by tackling tax avoidance - with the Conservatives claiming £5 billion and Labour claiming £7.5 billion.
As Labour bids to win voters’ confidence in its economic policy, why has it been so vague on one of its key promises?
Whether or not Ed Miliband makes it into Downing Street, he did today at least look the part of a Prime Minister in waiting.
The major measures promised by Labour in its manifesto for the upcoming General Election.