LONDON (Reuters) – Ryanair will not cancel British flights despite Monday’s start of what the boss of Europe’s biggest budget airline called a “rubbish” 14-day quarantine for international arrivals.

Ryanair, easyJet and BA-owner IAG have threatened legal action over the quarantine, which they say will cripple the British tourism industry.

Asked by BBC Radio whether Ryanair would cancel July and August flights if the quarantine remained in place, group CEO O’Leary said: “No, because the flights are full outbound of the UK. British people are ignoring this quarantine, they know it’s rubbish.”

The three airlines have sent a “pre-action protocol letter”, which can be followed by legal action, in the most vociferous clash between the UK government and industry since the pandemic began.

Airlines have warned it could deter British travelers, some of the top spenders, from flying to Europe and prevent Europeans from visiting cities such as London and Edinburgh and sites including Stonehenge.

There were 41 million visits to the UK in 2019, according to the national tourism agency, with visitors spending 28 billion pounds ($36 billion) and supporting millions of jobs.

The head of London’s Heathrow Airport, Europe’s busiest, warned that millions of jobs could be lost if aviation does not resume quickly.

The boss of easyJet said the industry could be left in tatters, with more job cuts needed from his airline, too.

The quarantine is designed to prevent a second surge of COVID-19 and in England a breach of the rules will be punishable with a 1,000 pound fine.

It will be reviewed every three weeks and “air bridges” that allow tourists to travel between two countries without needing to quarantine are being discussed with some top European destinations such as Portugal.

P&O Ferries said it had seen high levels of demand at Calais over the weekend as people tried to return before midnight although its efforts to return passengers were curbed by its need to limit customers on ships to allow social distancing.

($1 = 0.7875 pounds)