The legal action calls for transparency concerning the government’s traffic light system for travel. The group claims that ministers have not been clear about how the government has made its decisions regarding the categorisation of countries as red, amber or green.

Portugal was moved from the green list to amber list earlier this month, throwing the holiday plans of thousands of people into chaos.

Court papers are due to be filed today (Thursday) with health secretary Matt Hancock and transport secretary Grant Shapps named as defendants.

Ryanair and the Manchester Airport Group are calling on the government to publish the Covid-19 prevalence thresholds it deploys to determine whether destinations are classed as red, amber or green. It also wants transparency regarding criteria, advice or information that informs its decision-making.

India wasn’t moved to the red list until 20 April despite being home to the highly transmissible ‘Delta’ variant of Covid-19, with cases rising quickly since the beginning of April.

Michael O’Leary, CEO of Ryanair Group, said: “The UK’s traffic light system has been a complete shambles from the beginning. This go-stop-go-stop policy is causing untold damage to the aviation industry and frustrating and upsetting millions of British families when they see their holiday plans and family visits disrupted by the government’s mismanagement of international travel.

“We call on prime minister Boris Johnson to explain the scientific basis behind this system that the government seem to make up as they go along and to establish a data-driven transparent model that could restore confidence in air travel ahead of the very crucial peak summer months.”

Ryanair launched a flash sale for flights this week with fares as little as £4.99 one way to several amber destinations including Alghero in Italy, Lourdes in France and Santander in Spain

Charlie Cornish, MAG’s chief executive, said: “Recent developments suggest that the government is now unwilling to open up international travel by putting low-risk countries on the green list. For most countries, the traffic light seems to be stuck on amber for no obvious reason, despite having prevalence rates much lower than the UK.

“The government is not being open and we simply cannot understand how it is making decisions that are fundamental to our ability to plan, and to giving customers the confidence to book travel ahead.

“These issues must be resolved urgently – and ahead of the review point later this month – to allow everyone to understand how the system operates, and to create the opportunity for international travel to resume to the fullest extent possible over the summer.”

What is the traffic light system?

The government’s traffic light system categorises countries as green, amber or red.

Travellers returning to the UK from green list countries don’t need to self-isolate. However, they will still be required to carry out Covid tests before heading off on the trip and after arriving at their destination.

People arriving in the UK from amber countries need to quarantine at home for 10 days and take PCR Covid-19 tests on or before day two and on or after day eight after returning home.

Anyone arriving in the UK from a red list country needs to pay £1,750 to quarantine in a managed hotel for 10 days.