Fewer than 1.9 million flyers boarded a Ryanair flight in December, as the impact of Covid-19 on airlines continued over the Christmas period.
Ryanair said that its passenger number dropped by 83% last month, compared with the year before, in another major blow to the airline industry.
In December 2019, the airline carried 11.2 million travellers, it said in a message to investors on Tuesday.
At the same time, Hungarian airline Wizz Air announced that the number of passengers it carried in December had dropped 80% to 665,722.
Airlines have faced an immensely difficult 10 months as the Covid-19 pandemic first forced them to ground many planes, then to operate at significantly reduced capacity.
Across 2020, including the months before the virus hit its markets, Ryanair carried 52.1 million passengers, a third of its 2019 levels.
Passengers coming to the UK must self-isolate for 10 days, or self-isolate for five days and show a negative Covid-19 test.
Meanwhile, passengers flying from the UK are currently heavily restricted in their travel, due to a new strain of Covid-19 which is spreading in the country.
Last month Wizz Air and Ryanair, which are both listed in London, said they would remove the voting rights of their UK-based shareholders in a bid to remain majority-owned by EU shareholders.
This is a condition for an airline wanting to fly domestically within the European Union and the change was made necessary because of Brexit, the companies said.
Despite the Covid-19 pandemic, Wizz last month announced that it was adding six new routes to Turkey, Bulgaria, Spain and Greece from Doncaster Sheffield, Gatwick and Luton airports.
Ryanair also ordered 75 new Boeing 737 Max planes, as the troubled aircraft, which was involved in two fatal crashes, prepares to return to service in Europe.