Europe’s biggest low-cost airline has revealed major expansion plans for UK services next summer. But Ryanair is warning that unless a post-Brexit aviation deal is concluded soon “there will be no flights” after March 2019.

The Irish budget carrier will connect Stansted with Frankfurt, presenting a challenge to British Airways and Lufthansa. It will also fly to Weeze in western Germany, close to the Dutch border.

Other routes will serve Aalborg in Denmark, Oradea in Romania and Pardubice in the Czech Republic.

“This will take us to 25 million passengers across the three London airports,” said the chief executive, Michael O’Leary. But he said that the new routes would all be operated by planes and crews based in continental Europe.

The new services take advantage of “open skies”, the agreement that allows any European airline to fly between any two EU airports.

But Mr O’Leary has reiterated his concern about the lack of progress on an aviation treaty to replace it.

“The worst case scenario is becoming more and more likely,” he told Sky News.

“The onus is on the British government to deliver a deal. If there’s no deal by March ’19, Britain gets thrown out of the European Union, you’re out of open skies and there will be no flights.

“We haven’t got plan B. We haven’t even got plan A-and-a-half yet.”

The Ryanair boss singled out Theresa May for scathing criticism.

“I fail to see what she’s doing in Japan for three days at the moment, why she’s not in Brussels or in Frankfurt or in Paris which is where negotiations need to take place. There’s a crisis coming down the road here.

“Brexit is going to be a disaster for the UK economy, and she needs to be over there negotiating or at least removing these road blocks, not swanning around Japan drinking tea and sake.