London's Heathrow airport closed both its runways yesterday as snow brought major British air, rail and road transport systems to a halt, affecting millions.

Heathrow and London's three other commercial airports warned of severe delays and flight cancellations, many bus and train services were suspended and highway authorities warned of hazardous driving conditions in southern and central England.

More than 250 flights at Heathrow, the world's busiest international airport, were cancelled and a further 1,000 faced cancellation or delay, a spokesman for the British Airports Authority said. Heathrow handles 180,000 passengers a day.

Flights at Gatwick, Stansted and Luton to the north of London, and London City Airport were also widely affected.

Britain's Met Office said some parts of the country, including London and other parts of south east England, could be covered by up to 15 centimetres of snow by midday yesterday.

It issued a "severe weather" warnings for large parts of the country, with weather experts saying south east England was experiencing some of its worst snow since the early 1990s.

Tens of thousands of commuters were advised not to make the journey into work in the British capital and hundreds of schools were closed across the country. But many workers attempted to walk to their offices, trudging through thick snow.

Transport for London said all London buses - which would normally carry six million people on an average day - had been withdrawn from service "due to adverse weather and dangerous road conditions" and reported problems on all but one of the capital's eleven underground train lines.

The nation's railway operator, Network Rail, said many major routes had been severely affected with routes through Kent and Sussex in southern England the worst hit. The Southern and Southeastern rail firms suspended services into London.

So far, this winter has been Britain's coldest in more than a decade and forecasters expect the cold weather to continue for several more days with freezing winds blowing in from Russia.

The Highways Agency said drivers should avoid all unnecessary journeys and warned those who had to drive to proceed with extreme caution. "Be equipped with warm clothes, food, water boots, a torch and spade," it said in a statement.