Ryanair's chief executive today stepped up his warning about the risk of flights between Britain and Europe being grounded for days in the wake of Britain's exit from the European Union next March.
Michael O'Leary said there had been no assurances on the issue.
Mr O'Leary has long warned that Brexit could stop flights, taking a more pessimistic view than many of his rival airline bosses.
He said today that the risk of a no-deal Brexit was growing.
"Flights to and from Europe will be stopped in a hard deal Brexit," he told a news conference in London. "There is no assurance."
The CEO of rival carrier EasyJet said last week that he had been reassured by both Brussels and London that at least a basic agreement would be in place to enable flights to continue after Brexit next March.
Michael O'Leary said today that there were no guarantees.
He called on the UK Transport Minister Chris Grayling to prepare an alternative arrangement with the EU to ensure flights could continue in the event that there is no overall deal between Britain and the bloc.
"I do think it's possible that there will be an agreement on flights between the UK and the European 27 (countries), I'm just not sure that that will be reached in time for the first of April next year if there's a hard Brexit," he said.
Britain would struggle to agree separate deals with the 27 individual EU member states, he said, dismissing media reports that the country was looking to do this on flying rights, instead of one deal between the UK and the bloc.
He said a grounded flights scenario would not last more than "a couple of days or a couple of weeks".
"I think that politically it will get solved pretty damn quickly," he added.
Michael O'Leary said that the best case scenario for Ryanair was that Britain and the EU implement a transition period of 21 months, so existing flying rights could continue.
He made his comments as the airline gave details of its London Summer 2019 schedule, with 23 new routes through its four London airports - Stansted, Luton, Southend and Gatwick - planned for new year.
The 23 new routes including three from Stansted to Kiev, Lviv and Nantes; six from Luton to Alicante, Athens, Barcelona, Bologna, Cork and Malaga and 14 from Southend to Alicante, Bilbao, Brest, Copenhagen, Cluj, Corfu, Dublin, Faro, Kosice, Malaga, Milan, Palma, Reus and Venice.
From next summer, the airline will fly a total of 180 routes from its London airports.