British Airways announced it was canceling 170 incoming or outgoing flights on Friday, 84 of them domestic and the rest short-haul European ones.
BA was to operate all its long-haul services in and out of Heathrow, but some departing passengers were expected to face delays of several hours.
Heathrow's second-busiest airline, bmi, had scrapped eight flights by early Friday morning on top of 40 cancellations Thursday.
The chaos was expected to continue at other British airports, with Gatwick, Norwich, Southampton and Coventry among those predicting further delays.
However, BA said Friday morning that it had only canceled one of its Gatwick flights scheduled that day.
Airport operator BAA said it was providing a range of amenities for stranded passengers, including heated marquees outside terminals at Heathrow, with blankets and ponchos, sleeping mats, children's packs and food and drink.
BA also had buses to take as many as 3,000 people north from Heathrow to cities such as Manchester, Newcastle, Glasgow, Edinburgh and Aberdeen, where flights were going ahead as norma, reports APl.
At Heathrow the fog was expected to continue through the weekend, causing more potential delays for passengers making connecting flights.
British Airways said the fog was expected to continue to affect services at Heathrow, Gatwick and London City airports into Saturday night.
"It's bedlam," said Nicholas Velez, 23, from Washington, D.C. "The whole terminal is so packed you can barely walk."
With Heathrow hotels so full that even service rooms were occupied, Velez was one of the 500 people who slept in the chilly terminals overnight while waiting to rebook a flight home.
If one assumes that the two people who gave the interview indeed work for Russian special services, then they acted very unprofessionally and risky
Representatives of the Russian Defence Ministry said that the missile that shot down the passenger Boeing 777 aircraft over the Donbass on July 17, 2014, was manufactured in 1986