More than 60 flights were cancelled at Heathrow Airport because of Omicron infections causing staff shortages.
British Airways, Lufthansa and several others have been forced to cancel flights because of crew members or ground staff testing positive or isolating with further cancellations and delays possible today (Monday).
Staff who have not tested positive have been in contact with those infected but are then being forced to self-isolate.
Hundreds of flights across the world have been hit by rising number of Omicron infections with routes in the USA badly-affected. Leading US airlines Delta, United and JetBlue were also among those worst-hit, according to flight data.
JetBlue scrapped 10 per cent of its flights on Sunday. Delta cancelled five per cent and United axed four per cent according to FlightAware.
United warned earlier that a spike in Omicron cases had "had a direct impact on our flight crews and the people who run our operation", adding that it was contacting impacted passengers in advance of them coming to the airport.
American Airlines spokesman Derek Walls said the Christmas cancellations stemmed from virus-related sick calls.
Airlines around the world cancelled more than 4,500 flights during the Christmas weekend, as the Omicron variant created greater uncertainty and misery for holiday travellers.
Carriers globally scrapped at least 2,401 flights on Christmas Eve, according to a running tally on the flight-tracking website FlightAware, with 1,779 Christmas Day flights grounded worldwide.
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