Ryanair boss Michael O'Leary has criticised Theresa May's decision to visit Japan, claiming she should be in Europe working on Brexit negotiations.
The airline's chief executive accused the UK Prime Minister of "drinking tea and sake" when there is a "crisis coming down the road".
Mrs May met her counterpart Shinzo Abe for meetings focused on North Korea but also touched on a possible trade deal for when the UK quits the European Union.
Mr O'Leary said he did not have full confidence in Mrs May.
He told Sky News: "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 these negotiations need to take place.
"She's just come back from three weeks' holidays in the Swiss Alps. Now, everybody is entitled to their holidays, but there's a crisis coming down the road here (for) the UK economy in the next 12 months. Brexit is going to be a disaster for the UK economy.
"She needs to be over there negotiating or at least removing these roadblocks, not swanning around Japan drinking tea and sake."
Later on Thursday Ryanair will announce details of an increase in UK flights next summer, including several new routes.
Mr O'Leary said the Dublin-based carrier is "planning on the basis that somebody in the British Government will sort out Brexit", but added that he does not have "great faith in them at the moment".
He added: "We're going continue to invest in the UK and grow jobs here."
He warned that flights between the UK and the EU will be grounded in summer 2019 if no agreement on aviation is reached as part of the Brexit negotiations by September next year.
"We need to know six months before March 2019," he said.
"If there isn't the likelihood of an agreement there will be skin and hair flying.
"People trying to book their holidays for summer 2019 here in the UK this time next year ... the options will be driving to Scotland or getting a ferry to Ireland."
The single market for aviation, created in the 1990s, means there are no commercial restrictions for airlines flying within the EU.
The UK aviation industry has called for the liberal market to continue after Brexit.