Passengers stranded as fog hits flights in London

LONDON, Dec 21 (Reuters) Thousands of passengers were stranded today as heavy fog in southern England grounded hundreds of flights during one of the busiest travel periods of the year.

About 500 people spent the night at London's Heathrow after British Airways cancelled all domestic and some European flights to and from the airport due to poor visibility.

Airport operator BAA, which was bought by Spain's Ferrovial earlier this year, warned of overcrowding and delays for people trying to visit families for Christmas, or flying off for winter holidays.

At least 350 out of 1,300 flights at Heathrow were grounded.

''We are advising people to check with their airline before leaving their homes,'' a BAA spokesman said.

Heathrow, the world's busiest international airport, was shrouded in thick fog for a second day, prompting air traffic controllers to impose tight restrictions on flights.

The airport's Managing Director Mark Bullock urged passengers to check with airlines before going to the airport.

''It is very, very unfortunate that passengers are unable to travel, especially at this time of year,'' he told BBC radio. ''If they could avoid bringing friends and relatives with them that would help ease congestion.'' Flights were cancelled at other British airports, including 30 in Scotland, as the disruption spread.

Transport Secretary Douglas Alexander said airlines must ensure they have enough staff to keep passengers informed through call centres or Web sites.

''I appreciate just how difficult the situation is,'' he said.

''I would strongly urge them ... to take the necessary action both to get people in the air if they can, but if not, to make the right arrangements to make sure that there is the least inconvenience possible.'' Britain's Met Office said while the fog might lift from the Heathrow area later in the day, there was a continued risk of fog returning, right through to next week.

''There's always going to be a risk of fog over the next few days,'' she said. ''We're going to keep the high pressure over the southeast up to the weekend and into next week.'' Earlier this year, hundreds of flights were cancelled at Heathrow after strict security measures were enforced after police said they had foiled a plot to blow up airliners.

REUTERS SP HS1555