Senior Conservatives have said they would not be able to offer immediate tax cuts if they won the next election.
Mr Cameron has pledged to give a realistic view of the economy
Tory leader David Cameron told the BBC's Politics Show that tax cuts would be impossible at first because Labour had "left the cupboard bare".
Former Conservative leader William Hague said in a separate interview that any tax reductions had to depend on the state of the economy, not on politics.
A YouGov poll has placed the Tories 16 points ahead of Labour.
Mr Cameron said: "We have to recognise as an opposition that if we win the next election, it will be tough and there will not be some large kitty of money to spend and we will have to say no a lot, as well as hopefully being able to say yes to some of the things we want to do."
He went on: "There is no doubt that when it comes to the economy, that we are badly prepared.
"We have the highest tax burden in our history and we have got one of the highest levels of budget deficit of any developed country."
He said Britain would not be able to follow America's lead and cut taxes in order to stimulate the economy, because Britain's budget deficit as a share of national income was larger than America's.
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His views were echoed by Shadow Foreign Secretary William Hague on BBC1's Andrew Marr Show.
Mr Hague said: "We are not going to be a government that says 'oh, there's an election coming up so now we must have tax cuts even if the nation can't afford them'.
"But we are going to be a government that says that over the long term, over an economic cycle, we are trying to reduce the burden of tax on people."
Philip Hammond, Shadow Chief Secretary to the Treasury, told the Sunday Telegraph that the emphasis in the first term would have to be on building up reserves through efficiency savings.
He said: "We will make the savings, we will eliminate the waste and we will pile up the reserve so that at the following election, or before the following election we are able to show people where we will make the tax cuts."
But Chief Secretary to the Treasury Yvette Cooper said: "Another day, another Tory
"How can they deliver stability in the economy if they can't deliver stability
in their own policy?"
The YouGov poll for the Sunday Times suggested a two-point slump for Labour to 27% with the Tories on 43%.
An ICM poll for the News of the World suggested the Tories had gained three points to 40% with Labour down three at 31% and the Liberal Democrats down one at 20%.