Ryanair is launching a legal challenge against Britain over its "traffic light" system for international travel, hoping to force a relaxation of strict rules that threaten the summer holiday season.

The airline has teamed up with Manchester Airports Group (MAG) and the pair filed papers at England's High Court today to seek clarity over the transparency of the system, a MAG spokesperson said.

Other airlines are expected to join the legal action.

With just weeks before the peak July and August travel season when most profits are made, the aviation industry is worried about losing another summer to Covid-19 as the British government blocks most travel.

This will result in more job losses and financial strain for the sector.

The industry has repeatedly criticised the UK government's traffic light system for international destinations, saying it is unpredictable and doesn't make scientific sense. It says some low risk countries and islands should be open for travel.

A UK government spokesperson said it cannot comment on legal proceedings.

"We recognise this is a challenging period for the sector, as we seek to balance the timely reopening of international travel while safeguarding public health and protecting the vaccine roll-out," it said.

Britain allowed holidays again from May after months of lockdown, but discourages travel to popular holiday destinations, such as Spain, France, Greece and the US, classifying them as "amber" under its traffic light system.

This means travellers must quarantine for ten days on their return and take multiple Covid-19 tests.

Only a handful of places are classified as green, and none in the European Union after Portugal was removed at short notice earlier in June.

The court papers will argue the British government should clearly explain how it makes decisions on categorising countries, given the "dramatic" impact these decisions have on the aviation industry.

"The current opaque way that decisions are being made is undermining consumer confidence to book summer holidays and makes it impossible for airports, airlines and other travel companies to plan for the recovery of international travel," the companies said in a statement to Reuters.

The aviation industry had been counting on people in the UK to be at the forefront of the resumption in travel, given the country has one of the fastest vaccine roll-outs in the world.

Meanwhile, Ryanair has also called on the Irish Government to allow vaccinated citizens immediate access to their EU Digital Covid Certificates without further delay.

The airline said over a million fully vaccinated Irish citizens still await their Covid-19 certs and this is keeping them "locked up".

"All of Ireland's vulnerable groups are fully vaccinated and over 60% of adults have received a first dose vaccine, yet Ireland continues to impose the strictest travel lockdown in Europe," a spokesperson said.