Services to Morocco and Ukraine have been affected, as well as unspecified domestic routes from London, Manchester, Liverpool, Edinburgh, Belfast and Derry.

The row with the CAA centres on Ryanair’s UK subsidiary, which the Irish airline set up ahead of Brexit to maintain UK domestic flights after 1 January and to serve non-EU countries, including Morocco and Ukraine.

The CAA said Ryanair planned to use leased, foreign-owned aircraft for its UK operations. The CAA said only one of Ryanair’s entire fleet of more than 470 aircraft was registered in the UK and that leasing “undermines the competitiveness of the UK aviation industry”.

The airline claimed: “Only 10 days away from the end of the Brexit transition phase, the CAA’s director David Kendrick has inexplicably introduced new regulatory barriers in the way of Ryanair’s UK airline, forcing Ryanair UK to cancel 12 important routes from the UK’s regions at a time when they need reliable, low-fare connectivity more than ever before.”

However, the CAA said the decision to cancel flights “was taken by Ryanair alone”.

Paul Smith, the CAA’s consumers and markets director, said: “It is incorrect for the airline to state that the UK Civil Aviation Authority has changed its wet-leasing policy at short notice. 

“It has been our long-standing position that a UK airline with a significant presence in the UK, such as Ryanair UK does, should not rely heavily on using wet-leased, foreign-registered aircraft to undertake their operations. 

“Doing so undermines the competitiveness of the UK aviation industry and the effectiveness of the regulatory regime. This is a view shared by regulators around the world and has nothing to do with our preparations for the end of the transition period, which we have planned for extensively.”

TTG asked Ryanair for a response but it declined to comment further.