By Natalie Thomas

Ryanair believes Britain's courts will either strike down a 14-day quarantine for international travellers this week or the government will drop the measure before the end of June, boss Michael O'Leary told Reuters on Monday.

Britain introduced the 14-day quarantine on Monday in a bid to limit the spread of the novel coronavirus despite the threat of legal action from some of the largest airlines that fly in and out of one of Europe's biggest airline markets.

O'Leary said action proposed by British Airways-owner IAG and supported by low cost rivals easyJet and Ryanair seeking injunctive relief has a "high likelihood of success." Legal papers are due to be filed on Monday or Tuesday, he added.

"I think in their heart of hearts the government would like the courts to strike it down because it would get them off the hook. I think either the courts will strike it down this week or the government will quietly drop it before the end of June," O'Leary said in a Reuters TV interview.

O'Leary, who has become one of the most outspoken critics of the measure, calling it "completely useless and defective" on Monday, said it has not stopped British holidaymakers planning holidays abroad with bookings "building very strongly."

He said he expects flights from the United Kingdom to be heavily booked when Ryanair ramps its daily flight schedule back up on July 1 but that the quarantine would cause "untold devastation" to British tourism.

Inbound bookings into the UK in July are the only notable weakness Ryanair sees as confidence returns.

"It seems to be the usual peak summer holidays, families going abroad for some much needed sunshine and outbound bookings are very strong from Ireland, the UK, Northern Europe going to Mediterranean," O'Leary said, namechecking holiday destinations in Spain, Portugal, Italy and Greece.

(Writing by Padraic Halpin in Dublin; editing by Guy Faulconbridge)