A Conservative cabinet minister has admitted his party does not know where its planned £12 billion welfare cuts will come from.

Iain Duncan Smith, the Work and Pensions Secretary, said that the Tories had not yet "done the work" to calculate where the axe would fall.

Labour sources accused the Conservatives of treating voters like fools, and said the party should have worked out its plans before running for re-election.

The Tories have come under increasing pressure over the planned cuts to be benefits bill throughout the election campaign.

David Cameron was accused of lying to voters last week after he repeatedly refused to reveal what welfare payments would be cut.

Lib Dem leader Nick Clegg has accused the Tory Chancellor George Osborne of having a "dangerous" plan over the issue.

Both his party and the Democratic Unionists (DUP) have suggested that they could not do a deal with the Tories if they insist on taking a further £12bn from the benefits bill.

The Conservatives have called on voters to trust them on the issue and look at the party's record on benefits reform over the last five years.

But under sustained questioning over where the cuts would come from Mr Duncan Smith admitted that the party did not know.

He said: " We would have to have done the work on it. That's why.

"We would have had to reach agreement as to exactly where those are... as soon as we've done the work and had it modelled we'll let everybody know what that is."

Labour have claimed that the Tories will plan a £3.8bn raid on tax credits to meet the planned savings.

In a highly unusual move Danny Alexander, the Lib Dem Treasury Secretary, last week released details of what he said was an internal government document looking at the potential to cut Child Benefit and Child Tax Credit.

Labour leader Ed Miliband will today attack the Tories claiming they are the party of the privileged few as he highlights his vow to scrap the 'non-dom' rule that allows many of the richest in society to avoid tax on worldwide income.

He will say that "any government" he leads will implement the policy, suggesting it could be a dealbreaker in any formal or informal coalition negotiations.

He will say: "Let me be clear: any government I lead after this election will abolish the non-dom rule.

"The next Labour government will do what no government has done for 200 years. We will replace the non-dom rule with a clear principle: anyone permanently resident in the UK will pay tax in the same way."

In an interview Mr MIliband also disputed claims Labour would end up borrowing more money than a Conservative government.

The Labour leader said that he did not accept figures from the Institute for Fiscal Studies (IFS), which estimated that debt would be £90bn higher in 2019-20.

Earlier Mr Duncan Smith had told voters that a vote for Ukip was like writing a "suicide note" for eurosceptics' hopes of a referendum on Britain's membership of the European Union.

The ex-Tory leader, a prominent euro-rebel during John Major's time as prime minister, issued an appeal to those considering supporting Nigel Farage's party.

Backing Ukip would allow Labour to take power and deny the people a vote on the UK's relationship with Brussels, he said.

"I would simply appeal to them and say, honestly this is a risk that is no longer a protest, but it's like a suicide note," he said.

"The last thing I always say to Ukip people on the doorstep is that for 24 years I have campaigned for a referendum on the European Union.

"And the British people have wanted one. And they should stop and think because if they do not vote for the Conservatives, then basically they will have taken away that referendum from the British people for which they will not be forgiven.

"And I certainly won't forgive them."