A natural gas leak from a well in the British North Sea has forced the evacuation of drilling platforms for miles around, and plugging the leak could take months because of the danger of triggering a gas explosion.

Environmental experts estimated that the well, operated by the French oil giant Total, may be spewing greenhouse gases equal in impact to the two biggest U.S. coal-fired power plants or more than all of Norway. Natural gas is composed largely of methane, which is 21 times more potent a greenhouse gas than carbon dioxide.

Unlike the oil spilled from BP’s Macondo well in the Gulf of Mexico in 2010, the Total leak is primarily natural gas that dissipates in the air, especially in the usually windy conditions of the North Sea. But the gas in the well is known as sour gas because it contains toxic, flammable hydrogen sulfide as well as gas liquids that have created small surface sheens.

Drilling a relief well, one possible solution, would be difficult because any rig would have to keep its distance from the gas leak.

“This is another instance in which we see that the oil companies are not prepared for the worst-case scenarios,” said Frederic Hauge, president of the Bellona Foundation, a Norwegian environmental group.

The uncontrolled leak takes place amid controversy over how Britain should manage its aging offshore oil and gas fields. Just a week ago, British Chancellor George Osborne proposed about $4.8 billion in tax breaks to help oil companies dismantle old platforms and drill new wells. “Gas is cheap, has much less carbon than coal and will be the largest single source of our electricity in the coming years,” he said in his budget statement on March 21. He said the energy secretary would “set out our new gas generation strategy” in the fall.

Total said the leak began Sunday after work was done on an old well linked to a production platform in the Elgin field, 150 miles east of Aberdeen. By Monday morning, the company had evacuated all 238 people on board the platform.

Royal Dutch Shell partially evacuated a platform and halted drilling being conducted by the Noble Hans Deul rig about four miles from the Total well. It said Tuesday that it would move up plans for maintenance and shut down production on its Shearwater platform as a “precautionary” measure.

Total halted production from the Elgin, Franklin and West Franklin gas fields, sending the price of the company’s shares down 7.1 percent on the U.S. stock market. Total owns 46.17 percent of the fields and gets gas equivalent to about 60,000 barrels a day of oil.

“A surveillance aircraft flown over the area has confirmed a sheen on the water in the vicinity of the platform,” Total said in a statement Tuesday. But the company said the sheen was “related to drilling muds and/or light condensate associated” with the gas. It said that “preliminary assessments indicate no significant impact to the environment and dispersants are not considered necessary at this stage.”

Drilled in a little over 300 feet of water, the Total well reached far below the ocean floor and the reservoir set records for high-pressure conditions and high temperatures — about 190 degrees centigrade — when it was drilled in 2003, according to offshore-technology.com, an industry Web site. The field was discovered in 1991 by Elf Aquitaine, which was later acquired by Total.

Total officials told British news outlets that they were being careful to avoid igniting the gas. The worst accident in the North Sea took place in 1988, when 167 people died in an accident at the Piper Alpha oil platform.