Christmas travellers face a third day of misery as thick fog leads to more flight cancellations and delays.
A spokesman for airports operator BAA said that "fairly significant problems" were expected for Friday.

More than 300 flights have been cancelled at Heathrow, including all British Airways domestic flights, with 40,000 people expected to be affected.

Motorists are being warned to expect hold-ups on the roads and train firms are laying on extra services to cope.

Low visibility had caused air traffic control to place restrictions on flights landing and taking off at Heathrow, where 300 passengers slept at the airport overnight.

They were offered blankets, sleeping mats, food and hot drinks by staff.

One traveller said: "I've been here basically since Wednesday night, and I got rebooked three times and all my flights have been cancelled so far.

"So right now it's Friday morning and I don't know how much longer I'm going to stay here."

BA transported around 3,000 passengers to UK destinations from Heathrow by coach on Thursday.

And the airline was laying on similar services again to Newcastle, Manchester, Aberdeen, Glasgow and Edinburgh on Friday.

Gatwick, Manchester, Glasgow, Edinburgh, Norwich, Coventry, Cardiff and Southampton airports have been disrupted by the fog and the knock-on effects from Heathrow.

Geoff Want, BA's director of ground operations, said: "The weather across much of the UK is regrettably showing little sign of improvement for Friday and therefore we have no option but to make some more cancellations."

He apologised to customers and said that the airline was "working around the clock" to try to get people to their destinations.

BA announced it would be operating a reduced short-haul schedule because of the fog on Friday, and passengers on long-haul services were warned they may also face delays of several hours.

Backlog

BMI, Heathrow's second-busiest airline, has cancelled eight flights so far, adding to the 40 scrapped on Thursday.
The airline's chief executive Tim Bye said if necessary the airline would fly on Christmas Day to get people back home.
But Simon Baugh from airports operator BAA told the BBC's Today programme that the main problem at Heathrow was one of capacity.

He added: "It's the world's busiest international airport, we only have two runways, when if you compare our main competitors in Europe, Frankfurt has three, Paris has four, Amsterdam has five.

"We have fundamental capacity constraints at Heathrow."
Extra runway

With flights cancelled, many travellers have turned to trains to complete their journeys.
Virgin Trains, which runs services between London and Scotland, has lifted restrictions on saver tickets for Friday and is running seven extra trains to help ease the travel chaos.

GNER, which also runs between London and Scotland, has also announced it will be offering extra trains.

Network Rail said there were still some seats available on other services and advised passengers to contact National Rail Enquiries.

Edward Funnell, spokesman for the Association of Train Operating Companies, told the BBC's Today programme that rail services would cope.

"Many of the long-distance operators lay on extra services and we believe we will be able to cater for air passengers who wish to transfer to the trains, to get people away and home today but also tomorrow and Sunday."

Eurostar said its services to Europe were running as usual, but passengers were also warned to contact the company before travelling to check if seats were available.

Road jams

Meanwhile forecasters are predicting fog in England and Wales could persist during the holiday period.

BBC weather forecaster Darren Bett said "right until Christmas and even beyond there's the risk of fog".

Most of England and Wales will be affected, with only the north of England, the south coast and parts of Cornwall escaping the fog on Friday.

People who have opted to travel by car could also face jams on major routes as last-minute Christmas-shoppers and people travelling home for the festive break head for the roads.

The RAC said an estimated 18m people were expected to be driving over the next few days with the rush hour on Friday expected to start at 2pm and last until 7pm.

Delays are expected in Kent as the M20 is closed coastbound between Ashford North and Ashford South, because of a serious accident.