LONDON, Jan. 6 (Reuters) - Snow and freezing conditions brought transport chaos to Britain on Tuesday, shutting major airports, disrupting train routes and forcing hundreds of schools to close.

Severe weather warnings were in place across virtually all of Britain and in many areas people were advised to travel only if their journeys were essential after snow clogged roads and hit transport services.

Runways at major airports including Gatwick and Luton were closed due to heavy snow and Heathrow carried an advisory on its website, urging passengers to contact their airlines for the latest update.

The Met Office said 15 cm (6 inches) of snow had fallen in Manchester and western Wales, with 11 cm in Gloucestershire, 8 cm in Devon and 5 cm in Aberdeenshire.

Further heavy snowfalls were expected in the south and in London late on Tuesday and on Wednesday, with Berkshire, Buckinghamshire, Hampshire, Oxfordshire, Surrey and Wiltshire predicted to be worst hit.

Temperatures fell to as low as minus 15 C (5 F) overnight and the Met Office warned that as much as 40 cm of snow could fall in some areas as the weather front spreads south.

''A period of exceptionally heavy snowfall is expected with accumulations of 15-30 cm and perhaps in excess of 40 cm,'' the Met Office said.

''This is expected to cause widespread disruption to the transport network and could lead to problems with power supplies.''

Manchester Airport was closed until 1 p.m. because of heavy snow, and flights also were cancelled at Leeds Bradford, Liverpool and Glasgow Airports.

Bus services in Manchester were badly hit.

Many train services across Britain suffered cancellations and delays, with severely reduced services on the East Coast mainline from Scotland and northern England to London.

Motorists were advised to take care if they had to travel, and the Highways Agency said roads in high-risk areas would be salted continuously.

The Met Office has said it expected the freezing conditions -- caused by winds coming from the north and east instead of the more usual west and southwest -- to continue into late January after one of the coldest Decembers on record.