Le Stink arrives in London

The Met Office said the odour, described as being similar to manure, was being blown across Britain by an easterly wind. They say it originated in Europe and warned the stench could continue until Tuesday.

"We've been inundated with calls and we are warning people not to panic as we believe the smell is harmless," said a Met Office spokesman.

"We have had reports from across East Anglia, the south coast and London, although the smell does seem strongest around London because of the easterly winds bringing it in.

"When the wind blows from the west, it's coming from the Atlantic so it brings in virtually no pollution but when it is eastwards it is coming across land."

Met Office experts are now planning to run a series of computer simulations to pinpoint the source of the odour. One said: "This is likely to be a continuing problem, as there is not going to be a change in the wind direction until Tuesday at the earliest."

The Environment Agency said it was investigating the problem, and hoped to issue guidance to concerned Londoners later today. Thames Water said it had received dozens of calls, and staff as far afield as its Reading office had smelt the stench when arriving for work. A Thames Water spokesman said: "We checked all our sewage works and they are operating normally."

He added: "For a smell on this scale, we would have had to have a total breakdown of treatment works, and even then it would take a few days for the smell to appear."

Millions of people woke up to the smell and could not understand what it could be, blaming sewage works, gas leaks and even animals.

"I just walked out of my front door and the whole place stinks," said Javed Hussain from Twickenham, a caller to Nick Ferrari's LBC show.

City streets miles away from farmland reported experiencing the stench. Radio stations received dozens of calls from listeners from Suffolk, across London and as far west as west Berkshire-In Windsor, the smell lingered over the castle.

A spokesman at the town's tourist office said: "When I left home this morning the smell was virtually unbearable but we haven't had any complaints from anyone so far. I think the Queen is in. I hope she has her windows closed."

THE SCIENCE OF SMELL

HOW badly a chemical smells depends entirely on its size and shape, and how it fits in smell receptors in our nose. Gases which smell tend to be the ones which are heavier. This is why methane and carbon monoxide don't smell but sulphur dioxide and chlorine do.

The weight allows them to bind more effectively to olfactory receptors in our nose. Our sense of smell is 10,000 times more sensitive than our sense of taste, and is controlled by the receptors, which occupy a stamp-sized area in the roof of the nasal cavity.

Tiny hairs, made of nerve fibres, dangle from the receptors. If a smell, formed by chemicals in the air, dissolves in mucus, the hairs absorb it and excite olfactory receptors.