British Airways said it cancelled more than 800 flights during severe fog in the run-up to Christmas.
The airline said it was forced to ground the planes as long-haul flights were given preference over domestic and European short-haul flights.
It brought chaos to Heathrow and other airports and saw the number of passengers BA carried in the UK and Europe fall by 33,000 to 1,647,000 in December.
BA lost 34,000 passengers around the world last month as strong gains in Africa and the Middle East offset declines in the Americas and Asia Pacific.
It carried a total of 2,694,000 passengers in December and 27,975,000 between April and December - 573,000 more than the same period of 2005.
The airline was also hit by baggage problems at Heathrow last month which left thousands of passengers without their luggage. It started with a fault on a baggage belt at Terminal 4 and the chaos caused by the fog added to the disruption.
The fog and baggage crisis brought to an end a troubled year for BA in which it also bore the brunt of tougher security measures after an alleged terrorist plot to blow up transatlantic airlines was thwarted in August.
The disruption caused by the plot and new carry-on baggage requirements resulted in BA having to cancel around 1,270 flights. At one point the airline was struggling to reunite thousands of passengers with their luggage.
The August disruption cost BA around £100 million and it now faces another hefty bill as a result of the fog after it refunded thousands of passengers who fell foul of the restricted number of flights at Heathrow.
To add to the strain, the airline put passengers up at accommodation in the Heathrow area as well as laying on coaches to get passengers from the west London airport to UK destinations.
