People heading off for the great Christmas getaway are finding their travel plans in peril as rail failures and floods stalled journeys for millions.
Flood warnings across the UK are hampering the road network, while rail services have also been crippled.
A signalling problem in west London caused major delays on Saturday morning, with services in and out of Paddington affected, National Rail said.
Trains are now running normally to and from the station.
A major signalling problem caused by a fire on Friday that affected electrical cables and railside signalling equipment at Preston Park is also causing major disruption near Brighton.
Diversions are in place, trains between Brighton and Gatwick airport are travelling half-hourly and rail passengers are able to use buses.
The effects of the heavy rain in recent days also caused chaos on a number of rail routes.
Flooding between Taunton and Exeter St Davids and a signalling problem is causing disruption in the South West.
There are no trains between Plymouth and Tiverton Parkway or Taunton, and a replacement bus service will run from Exeter St Davids to Tiverton Parkway and Taunton, but is expected to be busy.
Further road replacements are being organised to run between Plymouth and Exeter St Davids, but this is expected to add another 90 minutes to journey times.
First Great Western and CrossCountry services are accepting each other's passengers, National Rail said.
A shortage of train crew is disrupting services between Bedford and Bletchley, with replacement bus services, and flooding is causing disruption between Liskeard and Looe in Cornwall, with bus services replacing trains.
A landslip has also caused rail problems in the Glazebrook area near Warrington.
The First TransPennine Express service from Liverpool Lime Street to Manchester Piccadilly will be diverted via Newton-le-Willows, and buses are replacing trains from Warrington Central to Manchester Oxford Street.
There is also a reduced service running from Manchester Piccadilly to Liverpool Lime Street.
First Great Western, which operates train services in and out of Paddington, warned that passengers should expect "significantly increased" journey times due to the signal failure at Hayes and Harlington in west London.
The rail company warned that because of the flooding disrupting services to the South West, services operating between London Paddington and Bristol Temple Meads towards Exeter St Davids, Plymouth and Penzance will terminate at either Taunton or Tiverton Parkway in both directions while services due to operate through to or from Penzance will start or terminate at Exeter St David's.
Flooding is also affecting the road network.
The A38 from Bodmin towards Exeter is closed between the A374 Plymouth road and the A3121 because of flooding, and is expected to reopen at 8am, the Highways Agency said.
The A19 is affected at the junction with the A139, with one lane on the southbound entry slip closed and the northbound exit slip road shut.
There are also delays on the A27 eastbound between B2144 and A285 near Chichester because of flooding.
The cancellation of the Heathrow Express rail caused havoc for people trying to get to the west London airport, on Friday forcing many of the 123,000 departing passengers to take the Underground instead.
Heathrow is due to handle a further 114,000 departures tomorrow and 88,000 on Christmas Eve.
British Airways will handle nearly 31,000 passengers on Christmas Day, with the airline carrying nearly 1.6 million in the period up to January 7.
Many of the estimated four million Britons travelling abroad for the holiday set off yesterday, with popular destinations including the Canary Islands, Tunisia, Cuba and Mexico.
From yesterday until January 3, a total of 700,000 will be leaving from Gatwick, 320,000 from Manchester, 300,000 from Stansted, 125,000 from Luton, 116,000 from Birmingham and 75,000 from Glasgow.
Between yesterday and Christmas, 52,000 will fly from Edinburgh and 56,000 from Bristol.
Popular overseas spots for new year revellers include Paris, Reykjavik, Rome, Dublin and New York.
Many hundreds of thousands will also travel across the English Channel by ferry or through the Channel Tunnel, with 160,000 heading off on the Eurostar.
The Highways Agency, which is responsible for all of England's motorways and major A-roads, said more than 250 miles of roadworks will have been completed before Christmas.
In addition, a further 57 miles of works will be suspended until January 2, but works remain in place at 19 locations covering nearly 100 miles.
The busiest roads this weekend are likely to be the M25, the M6 in the West Midlands, the M1 near Milton Keynes in Buckinghamshire, the M5 near Bristol and the M62 near Warrington in Cheshire.
VisitEngland said that over the festive period as many as 12 million Britons intend to take at least one trip in the UK involving an overnight stay.
These trips are likely to generate around £2.1 billion for the economy.
Two main line rail strikes that threatened travel chaos over the Christmas period have been called off.
But on the Underground, the train drivers' union Aslef is stopping work on Boxing Day in a strike that has already led to the postponement of the London football derby between Arsenal and West Ham United.
Apart from the Aslef Tube strike, the main train problem is likely to be engineering work on the West Coast line.
This will mean there will be no direct services from Euston station in London to Milton Keynes, Birmingham, Manchester, Liverpool or Glasgow on December 23 and 24.
Hourly services will run from Euston via an alternative route (Chilterns) to Nuneaton where passengers will be able to change for services heading to any and all the destinations mentioned above.
Direct services resume on December 27 but with two of the four lines closed up to and including New Year's Eve.
There is engineering work affecting a number of other lines, although Network Rail and the Association of Train Operating Companies have said that the number of replacement buses having to be laid on has almost halved compared with the 2011/12 festive period.