Mr. Clegg, a Liberal Democrat, told the BBC that the decision by the Conservative Mr. Cameron to veto proposed European treaty changes left Britain in danger of being “isolated and marginalized” in Europe. He added that if he had been in charge, “of course things would have been different.”

Mr. Cameron deployed his power of veto at a European Union summit meeting in Brussels, after failing to secure what he called vital safeguards for the health of London’s financial sector. But with the 26 other members of the European Union either agreeing to the proposed plan outright or saying they would put the matter before their Parliaments, Mr. Cameron’s veto on Friday left Britain alone on the margins at a time of great upheaval on the Continent, with the European Union struggling to resolve its financial crisis.

“This is the first veto in history not to stop something. The plans are going right ahead. It was a phantom veto against a phantom threat,” David Miliband, a former foreign secretary from the opposition Labour Party told the BBC on Monday. “David Cameron didn’t actually stop anything because the other 26 are going on and the provisions of the treaty would not have weakened our rights and freedoms one iota.”

The Labour opposition is likely to echo those complaints in Parliament, seeking to dent the enthusiasm of the dominant Conservatives. But the most serious political consequences of the veto concern the strained relationship between Mr. Cameron and Mr. Clegg.

On Friday, Mr. Clegg appeared to support Mr. Cameron’s decision, although he warned the Conservative Party’s anti-Europe wing against being too triumphant about the problems facing the European Union. But his stance hardened over the weekend, and on Sunday he appeared to have backtracked, or at least tried to finesse his explanation to show that was in line with his party’s pro-Europe principles.

In fact, Mr. Clegg told the BBC that when Mr. Cameron called him at 4 a.m. Friday with the news that Britain had vetoed the plan: “I said this was bad for Britain. I made it clear that it was untenable for me to welcome it.”

Mr. Clegg has already lost the confidence of many Liberal Democrats by appearing to betray the party’s position when he has supported the government on other issues, like increasing the amount of tuition colleges can charge.

After the summit meeting, many prominent Liberal Democrats went further than Mr. Clegg.

A former party leader, Paddy Ashdown, described Mr. Cameron’s veto as a “catastrophically bad move” and said it would do nothing to shield London’s financial district, the City, from future European regulations. “In the name of protecting the City, we have made it more vulnerable,” he said.

Lord Ashdown also warned that the move had alienated Europe in a way that would haunt the United Kingdom.

“The anti-European prejudice of some in the Tory party,” he said, “has now created anti-British prejudice in Europe.”

Mr. Clegg, a former member of the European Parliament, said he would now “fight, fight and fight again” to make sure Britain remained an influential force inside the European Union. He said he would resist “tooth and nail” efforts by some Conservatives to take the country completely out of the union, particularly since the United States has found Britain a useful conduit to Europe.

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

Mr. Clegg criticized Conservatives who had hailed Mr. Cameron as a “British bulldog” for his tough line on Europe.

“There’s nothing bulldog about Britain hovering somewhere in the mid-Atlantic, not standing tall in Europe, not being taken seriously in Washington,” he said.

To which one Conservative member of Parliament, Mark Pritchard, retorted, “Better to be a British bulldog than a Brussels poodle,” The Associated Press reported.

Mr. Cameron, meanwhile, was welcomed as a hero by his party’s anti-Europe right wing. “Up Eurs,” was the headline in Rupert Murdoch’s populist, anti-European tabloid newspaper, The Sun, along with a photograph of Mr. Cameron in a Churchillian bowler hat, holding two fingers up to Europe — the equivalent of an American middle finger.

“He did what I would have expected Margaret Thatcher to have done,” Andrew Rosindell, a Conservative member of Parliament, said approvingly.

But Kenneth Clarke, the Justice secretary and the Conservatives’ most prominent pro-Europe member, said in a radio interview that Mr. Cameron’s veto was a “disappointing, very surprising outcome.” He said he would be listening carefully to the prime minister’s statement in Parliament on the matter on Monday.

As upset as he is, Mr. Clegg said he did not want the coalition government to collapse.

“It would be even more damaging for us as a country if the coalition government was to fall apart,” he said. “That would cause economic disaster for the country at a time of great economic uncertainty.”

Alan Cowell contributed reporting.

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