The Chancellor said the Conservatives had accounted for only about a quarter of the funding needed to deliver their pre-election pledges.
He made the accusation as Labour released a 150-page dossier analysing recent Conservative pledges.
But Labour's claims sparked angry denials from the Opposition, who described them as "a dodgy dossier full of lies".
Mr Darling said its findings were based on the Tories’ own figures and Treasury-produced costings. The shortfall could only mean tax rises or deeper spending cuts, he said.
“The Tories have made over £45 billion of promises, but can barely explain how they can pay for a quarter of this,” Mr Darling said.
“This leaves them with a credibility gap of £34 billion.
“These are not long-forgotten promises from another time. All have been confirmed in the last two years. Most have been repeated in the last few months.
“You can’t fight an election on a nod and a wink, sometimes claiming you are committed to these promises, and when challenged claiming you are not.
“It’s now for them to say when and how they’ll be paid for. Or come clean and withdraw them.”
Mr Darling said Tory leader David Cameron had spent the past couple of years going around the country telling people "whatever he thinks they want to hear".
The cost of Conservative promises on tax and spending now came to £45 billion, he said.
The figures were based on the Tories' own costings except where there was "clearly more recent or credible numbers available", including Treasury figures.
"What is clear is that the Conservatives have so far only set out how to raise a fraction of the money needed to pay for their plans to introduce new tax cuts, reverse current tax changes and fulfil their spending commitments," he said.
"Just to meet those promises, let alone to cut the deficit faster, they would need new tax rises and deeper cuts to public spending elsewhere."
The Chancellor said he would tomorrow be presenting legislation to Parliament enshrining the Government's commitment to halving the deficit over the next four years.
The Labour dossier claimed that Tory plans for tax cuts would cost £21 billion a year by the end of the next Parliament, while reversing planned tax rises would cost another £13.3 billion.
Another £11.1 billion would have to be found to pay for spending commitments that have been made, Labour said.
The Tories' efforts to find savings in Whitehall expenditure would yield only £6.6 billion at best, while tax rises pencilled in by shadow chancellor George Osborne would raise £5.1 billion.
The alleged £34 billion black hole that would result would be on top of any cuts required to bring down the deficit "further and faster" than Labour, as intended.
Halving the deficit just one year faster than the Treasury already planned would require another £26 billion of spending cuts or tax rises, Labour said.
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