Speaking on the Andrew Marr Show, Clegg said he was “bitterly disappointed” that the prime minister refused to sign the treaty.

” I think now there is a danger that the UK will be isolated and marginalised within the European Union,” he said.

Clegg went on to warn that if Britain left the EU altogether, as some Tory eurosceptics want, the country would be disregarded by its American allies.

“A Britain that leaves the EU will be considered irrelevant by Washington and will be a pygmy in the world when I want us to stand tall in the world,” he said.

However Clegg’s comments were a u-turn from his statement on Friday, when the deputy prime minister said:  “The demands Britain made for safeguards, on which the Coalition Government was united, were modest and reasonable. They were safeguards for the single market, not just the UK.”

On Friday, Cameron refused to sign a treaty which would have imposed closer EU control over national budgets. The prime minister had called for safeguards to protect the City of the London, but Germany and France blocked his request.

It has been suggested that Clegg’s turnaround came after Lib Dem Business Secretary Vince Cable threatened to quit unless Clegg spoke out.

Former Lib Dem leader Paddy Ashdown is also said to have weighed in, describing Cameron’s decision as “catastrophically bad”, and telling Clegg the Coalition had to stand up to the b******s on the Tory Right.

Today, Cameron will face the Commons to defend his move and is expected to make the case that Britain could not sign the treaty without  “reasonable” safeguards for national interests.

Meanwhile a poll in The Times indicated that most people support the prime minister’s decision, with 57% agreeing agreed that Mr Cameron was “right to exercise Britain’s veto”.