A nasty gas leak from a factory in France has invaded Britain, with horrified residents in the South East flooding police with calls as the obnoxious odour left them gagging. Many feared it was so bad it may be poisonous. But health officials traced the rotten smell to a leak at a factory in Rouen, Normandy, about 70 miles west of Paris. The leak, from French chemical firm Lubrizol, released a colourless gas called mercaptan, or methanethiol - listed in the Guinness Book of Records as "the world''s smelliest substance", reports The Mirror. It is one of the main chemicals responsible for bad breath and flatulence and occurs naturally in humans. James Boyes, 42, of Dover, said the stench was "truly, truly awful". He said it was like the worst flatulent person ever standing with their back turned towards you. People living in Hastings, Eastbourne, Brighton, Folkestone, Dover, Deal, Tunbridge Wells, Maidstone and Ashford fell victim to the pungent gas. Even parts of South East London, including Croydon, Bromley, Sidcup and Lewisham, were not spared. Later reports said it had wafted as far as Reading in Berkshire. A statement from the Seine-Maritime prefecture in north east France claimed the concentration of the gas was very low, but admitted that "a large number of people have been inconvenienced" - including most of Paris. They added that the leak should be plugged by tomorrow. In the meantime people were advised to keep doors and windows closed. The Health Protection Agency said the unpleasant odour of the gas may make some people feel slightly nauseous, but it will dispel naturally.