The Great Getaway begins

With state schools breaking up today, the main routes out of the capital are expected to see heavy traffic from late morning, reaching a peak at about 4pm.

The usual bottlenecks are likely to be made far worse by dense fog to the north of London, the Met Office warned. Visibility could be cut to less than 100 yards.

A spokesman said: "The worst of the fog conditions are likely to be in the morning across the home counties north of London up to the Midlands. The worst conditions are when you get very dense patches of fog and you drive in and out of it. Drivers will have to take care.

"We will be lucky if temperatures get above freezing where there is fog. In the rest of the region it could get up to 5C."

Barry Gromett from the Met Office added: "It makes driving conditions rather more tricky than usual. It could get below 100metres visibility but it will disperse during the day."

Major routes that could be affected by the fog include the M40, the M1, the M11 and A1. The worst congestion is expected to be around the big shopping centres such as Brent Cross, on the M25 and on the busiest arterial routes such as the A1 Archway Road and the A40 Western Avenue.

On the railways all engineering work has been suspended until late tomorrow night to help reduce delays on what will be the network's busiest day of the year, with a total of four million passengers.

However, the work will start again on Sunday with passengers forced to board replacement buses on some of the country's most important routes such as the East Coast Main Line from London to Edinburgh and the West Coast Main Line from London to Glasgow.

London's airports - particularly Heathrow - are expected to be severely congested for the next four days as record numbers of Londoners head off for destinations in the sun and ski resorts.

There is also the possibility of delays at Stansted if the fog persists.

However, not everyone will be able to escape this Christmas. According to the TUC, a record 135,000 employees now work on Christmas Day, twice the number a decade ago. Almost 50,000 will be on duty on NHS hospital wards, about 12,000 in small shops and petrol stations and another 12,000 in pubs, bars and restaurants.