A cloud of harmless gas smelling of sweat and rotten eggs has leaked out of a chemicals factory in northwest France and wafted across the English Channel as far as London.
The leak occurred on Monday morning at a Lubrizol France plant near Rouen, 120 km northwest of Paris, and winds blew the invisible gas cloud south over northern France on Monday night and then up into England on Tuesday (overnight, NZ time).
The fire brigade in the county of Kent, southeast of London, warned residents to keep their doors and windows closed due to a gas cloud it believed had come from France and London police said it had reports of an acrid smell in southeast London.
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.
Internal operations director Pierre-Jean Payrouse said the company was battling to plug the leak, as the cloud spread over some 350 km, but said it might take until the evening. The cause of the leak was still unknown.
"It's not so much a leak as a product that has decomposed, which smells very bad and which is escaping," Payrouse told RTL radio. "An investigation is under way (into what happened) but our priority is to deal with the problem."
The gas, which is non-toxic but is flammable in strong concentrations, prompted a flood of phone calls to emergency services in France in the early hours of Tuesday.
The Paris police department issued a statement saying the gas posed no health risks but warned that it smelled like a mixture of "sweat, garlic and rotten eggs".
A French Cup football match between Rouen and Olympique Marseille had to be postponed because of the stink, the French federation said.
A local news website said the gas had caused migraines, irritations and nausea among some residents of Rouen.
Britain's National Grid, which receives emergency phone calls when people smell gas, said it was experiencing a large volume of calls, as did smaller gas providers across southeast England.
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 US 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