O'Leary, group chief executive of Europe's largest low-cost carrier, was speaking hours ahead of the announcement of new UK COVID-19 support measures and weeks ahead of the launch of a European Union plan to coordinate travel restrictions in Europe.

Failure join the EU plan and provide a proper furlough scheme to pay those whose work has dried up due to COVID-19 "will mean literally hundreds of thousands of job losses this winter," O'Leary told Sky News in an interview.

British finance minister Rishi Sunak on Thursday was due to announce a replacement for the Coronavirus Job Retention Scheme, which supported 8.9 million private-sector jobs at its peak in May, as part of a series of new COVID-19 support measures.

O'Leary said if there was no "proper" replacement, staff not required would be put on unpaid leave. Job losses cannot be ruled out, but Ryanair is "desperately working with our people to prevent them," he said.

O'Leary also called for the scrapping of the United Kingdom's air passenger tax for 12-24 months.

The EU travel scheme, due to be finalised in mid-October, would set out common COVID-19 travel rules across the continent and impose restrictions on regions with high COVID-19 rates rather than countries.

O'Leary, who earlier this month described the winter as a "write-off", said Ryanair's forward bookings for November and December are around 10%, around a quarter of the normal level for this time of year.

He said British airlines in general were likely to fly around 30-40% of their normal capacity this winter and said bookings were now around 20-30% of normal levels.

(Reporting by Conor Humphries; Editing by Mark Potter and Jane Merriman)