LONDON – Doubts resurfaced Monday over Europe’s ability to solve its debt crisis and rescue the imperiled euro, as investors worried that plans for closer fiscal unity will bring little immediate relief and Britain warned the deal could face new political hurdles.

British Prime Minister David Cameron was the only leader among the European Union’s 27 members to refuse last week to join a plan under which nations submit their budgets for central EU review and limit the deficits they can run.

As the rift between Britain, which has its own currency, and the 17 euro nations fed uncertainty about the deal’s implementation, ratings agencies Moody’s and Fitch warned the plan would make little difference.

The summit produced “few new measures,” and Europe remains in a “critical and volatile stage,” Moody’s said in a published report. It noted that the pact does not address Europe’s immediate problem: the crushing debt loads of some nations and their rising borrowing costs.

The agreement “kicks off a process that has a chance of solving the next crisis, not this one,” warned Guy LeBas, chief fixed income strategist at Janney Montgomery Scott.

Stocks plunged and the euro hit a 10-week low against the dollar as market confidence in the plan and Europe’s ability to end the crisis ebbed.

On Wall Street, the Dow Jones industrial average dove as many as 243 points before closing down 163, while European stocks also closed sharply lower. Yields on Italian bonds rose to 6.76 percent, closing in on the 7 percent level that forced fellow eurozone nations Greece, Ireland and Portugal to take bailouts.

Cameron defended his rejection in the House of Commons, telling U.K. lawmakers the fiscal pact that envisions using the EU’s executive arm as a budget watchdog could face even more political hurdles.

“The choice was a treaty without proper safeguards or no treaty, and the right answer was no treaty. It was not an easy thing to do, but it was the right thing to do,” Cameron said.

The British leader was greeted with cheers from his own euroskeptic Conservatives but jeers from opposition lawmakers, who worry Britain will find itself sidelined from key European decisions.

In Washington, Secretary of State Hillary Clinton backed Britain’s position, telling reporters “the role that the U.K. has played in Europe will continue.”

But French President Nicolas Sarkozy blasted Britain for dividing the continent. “There are clearly two Europes,” Sarkozy was quoted as telling Le Monde newspaper.

In Italy, one of the continent’s most troubled economies, workers angry about government austerity reforms went on strike and held nationwide rallies.

Strikes idled some Fiat auto plants and forced Milan’s famed La Scala opera house to cancel a performance. It was the first of days of union walkouts and demonstrations against spending cuts and tax hikes that Italy’s new technocratic government is seeking to restore investor confidence.

Unions say Italy’s austerity measures are hitting too hard on pensioners and workers and not hard enough on the wealthy. “Fairness, fairness!” shouted workers marching in Florence.

Twenty-three European countries – including Italy – have said they are in favor of the fiscal pact announced Friday in Brussels, while three more say they are open to the idea.

Under the deal, a central European authority would oversee their future budgets and impose tougher spending controls. The participants would also agree to automatic penalties if countries spend too much.

Markets had initially rallied Friday on news of the deal – despite Britain’s refusal to take part. But that optimism soured Monday as traders sought more short-term support for European financial markets.

They were also disappointed that the European Central Bank sharply cut back its purchases of government bonds to only $841 million last week, underlining the bank’s stance that indebted governments should dig out of their own debt problems.

Credit ratings agency Fitch said, “It seems that a ‘comprehensive solution’ to the current crisis is not on offer.” Moody’s warned it still plans to review all EU governments’ ratings for possible downgrades during the first three months of 2012.

Cameron’s decision to veto an EU treaty to solve the crisis has angered European counterparts – and will cause complications over how the pact is implemented and monitored.

Cameron insisted Monday that his stance was aimed at sheltering London’s financial services industry from burdensome new EU regulation.

“This is new territory and does raise important issues which we will want to explore with the ‘euro plus’ countries,” the British leader said, referring to the block of 17 eurozone nations and nine other EU countries who say they’ll either agree to or consider the new pact.

Even though the U.K. is not in on the plan, it can still torpedo it, since it does have a say in how EU-wide institutions are used.

“In the months to come, we will be vigorously engaged in the debate about how institutions built for 27 should continue to operate fairly for all member states, Britain included,” Cameron added.

At issue is whether the countries that join Europe’s new fiscal pact can use EU institutions – such as the European Commission, the EU’s executive based in Brussels – to implement their deal. Britain’s resistance will certainly keep the plan from any fast-track approval.

In Athens, Greece’s finance minister said the coalition government will cut spending further but won’t impose new taxes as the country heads into a fourth year of recession.

Finance Minister Evangelos Venizelos added that talks with international debt inspectors on a second bailout were at a “difficult and critical” stage, speaking after meeting with officials from the European Commission, the European Central Bank and the International Monetary Fund.

“It’s not a question of whether we can impose more taxes – we cannot. It is not a question of whether we must cut spending more – we must,” Venizelos said.

Greece, the country that kicked off the European debt crisis, is negotiating the details of a second bailout loan, this one at $174 billion.