The UK's coronavirus quarantine will be struck down by the courts or dropped within weeks as some of Europe's biggest airlines prepare to file a legal challenge by the end of tomorrow, Ryanair boss Michael O'Leary has said. 

The 14-day quarantine for international arrivals, introduced today, has led to the most vociferous clash between a major industry and Prime Minister Boris Johnson's government during the Covid-19 crisis. 

By insisting on the quarantine despite public pleas from airline veterans such as IAG's Willie Walsh and Michael O'Leary, Britain has united some of the fiercest rivals in aviation who are already reeling from a sudden global halt in air travel. 

Ryanair Group chief executive Michael O'Leary told Reuters he hoped a court would hear an emergency legal challenge by the end of this week to halt the quarantine - imposed just as most European countries were reopening their economies. 

"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," O'Leary said in a Reuters TV interview.

"I think either the courts will strike it down this week or the government will quietly drop it before the end of June," he added. 

He said legal action, proposed by British Airways and supported by low-cost rivals Ryanair and EasyJet, seeking injunctive relief has a "high likelihood of success." Legal papers are due to be filed today or tomorrow, he added. 

Slow to introduce lockdown measures and with one of the highest death tolls in the world, at more than 50,000, Britain argues a quarantine is needed to prevent a second surge of Covid-19. 

The UK quarantine imposes fines of up to €1,000 for any breaches. 

Ryanair, British Airways and EasyJet believe the measure is ineffective as passengers can still board trains and busses once they leave an airport. 

Willie Walsh, the head of BA and Aer Lingus-owner IAG, said the issue could be resolved quickly if they could prove it was an irrational piece of legislation, while EasyJet boss Johan Lundgren said the three companies had a strong case. 

"This is something that has been rushed through. It's not in proportion," Lundgren told Sky News. 

The quarantine will be reviewed every three weeks, the government has said. Portugal's foreign minister Augusto Santos Silva has said "air bridges" that allow tourists to travel between two countries without needing to quarantine are also being discussed. 

Michael O'Leary said Britons were booking outbound flights for holidays despite the quarantine, but Europeans were not coming to Britain.

The airlines say the quarantine measures are more stringent than those imposed on people suspected of being or confirmed to be infected by the coronavirus who are asked to isolate and do not face criminal sanctions.

Their letter also said it was "illogical and irrational" to impose quarantine on people arriving from EU countries that have lower infection rates than Britain.

EasyJet CEO Johan Lundgren

The UK quarantine plan was rushed into existence, is out of proportion and should be challenged in the courts, according to the head of EasyJet. 

Chief executive Johan Lundgren said he believed EasyJet, Ryanair and British Airways had a good chance of winning any legal battle and warned that if the measure stayed in place for a long time he could have to make further job cuts. 

"We think that there's enough evidence and there’s a strong case here that this should be challenged by the courts. This is something that has been rushed through. It’s not in proportion," Lundgren told Sky News today. 

EasyJet is already planning to cut 4,500 jobs or 30% of its workforce because of the coronavirus crisis, and Lundgren said quarantine would make the situation even worse. 

He said he hoped the threat of legal action would push the UK government to replace the measure with "a targeted approach that is based on the solutions of air bridges", which allow people to travel freely between countries with low infection rates. 

When asked if the quarantine could lead to more job losses, he said, "I fear so...I think and I fear unless there is a change to this (the quarantine rule), that the aviation industry as we know it here in the UK will not be in tact."

Meanwhile, the chief executive of London's Heathrow Airport said that hundreds of thousands of jobs, if not millions, could be lost in Britain if aviation is not able to resume quickly.

"We cannot go on like this as a country," Heathrow CEO John Holland-Kaye said.

"We need to start planning to reopen our borders. If we don't get aviation moving again quickly, in a very safe way, then we are going to lose hundreds of thousands if not millions of jobs in the UK just at the time when we need to be rebuilding our economy," he added.