Ryanair said Omicron travel restrictions in the UK and across continental Europe caused its passenger numbers to fall to its lowest level since July.
The budget airline said it served 9.5 million customers in December, a decline of 7 per cent from the 10.2 million people it flew in November before the latest round of coronavirus testing requirements and curbs were imposed for travel.
Ryanair ran 62,200 flights last month, with a load factor – a measure of how well an airline fills its planes – of 81 per cent, down from 86 per cent in November.
Shortly before Christmas, the group had issued a warning over profits as it slashed its passenger forecast for December and January due to the Omicron restrictions.
The Irish airline had previously expected to fly between 10 million and 11 million passengers in December.
It also slashed its guidance for January by 33 per cent as restrictions were imposed on British passengers flying to Germany and France and all EU passengers to Morocco.
In the run up to Christmas, France banned all non-essential travel from Britain, citing concerns about the rapid spread of Omicron across the UK.
Passengers have to provide a compelling reason to enter under the rules, which include exceptions for French citizens and their spouses.
Ryanair cut its full-year results guidance to a net loss of between £209 million (€250m) and £375 million (€448m) from a previous net range of between £83 million (€99.2m) to £167 million (€200m).
On a year-on-year basis, the no-frills airline reported an increase in passenger numbers from 1.9 million in December 2020, when there were strict restrictions to control last winter’s wave of the pandemic.
In early December, the UK announced a new set of rules that require all travellers over the age of 12 to show a negative Covid test to enter the country, regardless of vaccination status. A further test, which must be a PCR, has to be taken within two days of arrival.
Travel industry leaders have called on ministers to scrap the testing rules which they say are scuppering the sector's recovery.
As well as increased travel restrictions, the UK ministers imposed a set of “Plan B” measures in December to limit the spread of Omicron. Face masks were made mandatory in most public indoor settings, people were ordered to work from home where necessary and travel certificates showing vaccination or a negative test result were ushered in for nightclubs.
Despite the curbs, Covid cases surged over the Christmas and New Year period and, on Tuesday, the UK declared a new daily record of 218,724 infections.
Prime Minister Boris Johnson said on Tuesday he hoped to “ride out” the wave of Omicron without lockdown measures despite the NHS coming under significant strain.
He is expected to make a further announcement about Covid rules after a Cabinet meeting on Wednesday.