A cloud of harmless gas smelling of sweat and rotten eggs leaked from a chemicals factory in northwest France and drifted across the English Channel as far as London on Tuesday.
The leak occurred on Monday morning at a Lubrizol France plant near Rouen, 120 km (75 miles) northwest of Paris, and winds blew the invisible gas cloud south over northern France on Monday night and then up into England on Tuesday.
The fire brigade in the county of Kent, southeast of London, warned residents to keep their doors and windows closed due to the gas, which may make some people feel nauseous, and police said they had reports of an acrid smell in the capital.
Lubrizol France, which makes additives for industrial lubricants and paint, said the gas was mercaptan, also known as methanethiol, a colourless additive used in natural gas because its sulphurous smell enables gas leaks to be detected.
It was due to start an operation on Tuesday evening to stop the fumes, a process that could take hours or days, Pierre-Henry Maccioni, head of the Seine-Maritime regional government, said.
"It's not so much a leak as a product that has decomposed, which smells very bad and which is escaping," the firm's internal operations director, Pierre-Jean Payrouse, told RTL radio.
"An investigation is under way but our priority is to deal with the problem."
London tabloids, quick to seize on historical animosity between the British and French, went to town with the whiff. A Daily Mail headline lambasted a "French stench" while an article in the Sun cited a "mystery pong" that was "blamed on France".
Meanwhile, authorities in France and Britain assured affected residents the gas was not a health threat.
The Paris police department issued a statement saying the gas posed no health risk but warned that it smelled like a mixture of "sweat, garlic and rotten eggs".
A French Cup soccer match between Rouen and Olympique Marseille had to be postponed because of the stink, the French federation said.
The gas, which is non-toxic but flammable in strong concentrations, prompted a flood of calls to emergency services in France, with the Interior Ministry imploring concerned residents to stop phoning, so as not to overwhelm the system.
Britain's National Grid, which receives emergency phone calls when people smell gas, said it had received a record number of phone calls on Tuesday, with more than 100,000 calls registered by 1400 GMT, 10 times its normal call volume.
London's Metropolitan Police tweeted: "We are aware of reports of a strong, noxious, gas-like smell in some South East London boroughs. No risks to public."
Ohio-based Lubrizol, founded in 1928 and part of U.S. conglomerate Berkshire Hathaway Inc since 2011, has production facilities in some 19 countries.
Payrouse said the last time the company had experienced a similar incident was in the late 1980s.
An environmental safety consultant said cold weather and weak winds had probably helped the gas cloud reach Britain.
"The stable weather conditions and low temperatures at the moment ... mean that the smell is drifting across the UK, a distance of over 200 miles (320 km) from the source," Tony Ennis, technical director at Haztech Consultants, said.