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By Simon Browning & Daniel Thomas
BBC News
EasyJet has cancelled around 100 flights on Monday, including 62 from the UK, blaming higher than usual levels of staff absence due to Covid.
It said it had tried to tackle the problem by using standby crew, but also decided to make "to make some cancellations in advance".
It comes amid rising demand for travel as the Easter school holidays begin.
Airports are also facing high Covid-related absences that have led to long queues for security and check-in.
EasyJet, which is one of Europe's biggest airlines, said the cancellations were a small part of its schedule on Monday, which is around 1,645 flights.
"As a result of the current high rates of Covid infections across Europe, like all businesses easyJet is experiencing higher than usual levels of employee sickness," a spokesman said.
"Unfortunately it has been necessary to make some additional cancellations for today and tomorrow. We are sorry for any inconvenience this may cause."
Affected customers have been contacted and have the option to rebook on alternative flights or receive a voucher or refund.
Airports have also seen staff shortages at time when demand for travel is picking up following the end of travel restrictions.
Passengers at Manchester Airport have faced long queues for check-in and security, leading some to miss their flights at the start of the Easter holidays.
Some travellers landing also complained about extensive waits in the baggage reclaim halls.
On Sunday, traveller Donna Mayfield told the BBC the situation was "horrendous", while another said they had seen "customers and staff in tears".
Manchester Airport has apologised and admitted passengers' experiences fell "below the standard we aim to provide".
"Our whole industry is facing staff shortages and recruitment challenges at present, after the most damaging two years in its history," a spokesperson said.
"The removal of all travel restrictions after two years, coupled with the start of the summer travel season, has seen a rapid increase in passenger numbers, which is putting an enormous strain on our operation."
Covid-related absences have also hit Heathrow Airport's operations in recent weeks.
It told the BBC there had been long queues to enter the airport on Sunday morning at Terminal 2, although the situation was quickly brought under control.
The airport attributed the congestion to Covid checks required by destination countries and "high passenger volumes".
But there were also reports of staff shortages and problems with the e-gate passport checks, with some travellers saying they had waited hours to take off.
Gatwick told the BBC it was also having a busy weekend. It has not seen big queues in its security area, but said that some check-in areas run by individual airlines had been very busy.