Large swathes of England were engulfed by a foul-smelling gas cloud that wafted over from France, triggering thousands of calls to the emergency services and the National Grid.
People in Kent, Sussex and Surrey first reported the smell after a discharge of the gas mercaptan from a factory in the northern city of Rouen drifted across the English Channel.
By the afternoon the unpleasant aroma – similar to "rotten eggs" – had spread further up the country, with reports coming in from Oxfordshire and even as far north as Northampton in the East Midlands.
The National Grid, which normally deals with 10,000 calls a day, was inundated with more than 100,000 calls.
Mercaptan is added to municipal gas to alert people to gas leaks, among other uses, but is not toxic.