Challenged by environmentalist protesters, Prime Minister Tony Blair has announced that Britain will decide next summer whether to reverse its opposition to building new nuclear power stations.

The announcement Tuesday reflected a nascent European debate that could presage a dramatic shift in energy policies. Finland has already broken ranks with the opposition to nuclear power that has seized much of the Continent since the 1986 Chernobyl disaster. And while France derives about 80 percent of its electricity from nuclear power, countries like Germany and Britain may be poised re- evaluate their pledges to phase out nuclear power by the early 2020s.

"The issue back on the agenda with a vengeance is energy policy," Blair said. "Round the world you can see feverish rethinking. Energy prices have risen. Energy supply is under threat. Climate change is producing a sense of urgency." The speech came only two years after the British authorities resolved to increase the use of renewable sources such as wind power to 10 percent of the country's needs by 2010 and 20 percent by 2020. The authorities also said Britain's existing coal and nuclear power stations, which provide around a third of the country's electricity, would be phased out by 2023.

At the time, the government left open the possibility of building new nuclear power stations a move that has divided environmental groups and is opposed by some members of Blair's Labour Party.

"The future is clean energy," Blair said. "And nations will look to diversify out of energy dependence on one source."

Warning that renewable energy sources could not replace all the electricity generated by coal and nuclear power plants, Blair said Britain's latest policy review "will include specifically the issue of whether we facilitate the development of a new generation of nuclear power stations." As Blair spoke at an employers' meeting here, two men wearing fluorescent yellow jackets over dark business suits clambered into the steel rafters of the auditorium to launch a protest on behalf of the Greenpeace environmental group.

They carried banners saying "Nuclear: wrong answer" and scattered similar messages on ticker tape onto the crowd below. Greenpeace said the protest was designed to launch a "fightback against a new nuclear era" by preventing Blair from speaking.

The protesters refused to abandon their perches in the roof beams, insisting that they wished to make a 10-minute speech to participants in the annual meeting of the Confederation of British Industry, a leading employers' group.

"I'm not prepared to accept that," said Digby Jones, the head of the Confederation. "I don't give in to ultimatums." Blair, regarded as an undeclared supporter of nuclear power, was forced to address business leaders in a cramped side room. "This is going to be a surreal occasion," Blair said. "I'm going to give this speech if it's the last thing I do." "Like most tough issues, what we actually need is an open and democratic debate, not one conducted by protests and demonstrations to stop people having the freedom to express their views," he said.

The two protesters, identified by Greenpeace as Huw Williams and Nyls Verhauelt, had apparently infiltrated the building with unauthorized identification passes, the organizers said. The two were later arrested.

The Confederation of British Industry acknowledged that security around the prime minister had been compromised, and only months after the July terrorist bombings. Another speaker at the annual gathering was Ian Blair, the head of London's Metropolitan Police.

Britain's looming energy crisis is depicted here as potentially acute because the country, which long relied on its North Sea oil and gas reserves, has become a net importer of both, provoking concerns that Britons will consume more energy than they can produce or afford to import. Government officials say they are worried that Britain could become reliant on politically unstable countries for supplies of natural gas.

Opponents of nuclear power argue that it is costly, potentially dangerous, vulnerable to terrorist attack and dogged by the vast costs and difficulties of nuclear waste disposal. Advocates maintain that nuclear power plants produce clean and cheap fuel, reducing emissions of so-called greenhouse gases.

In the United States, no new nuclear plants have been ordered since the Three Mile Island accident in Pennsylvania in 1979. Nuclear power contributes around one-fifth of that country's electricity needs. But in Europe, the nuclear debate propelled by concerns over future shortages of oil and gas, pollution and high costs has begun to accelerate.