Tens of thousands of jobs in the aviation sector are at risk unless the Government further relaxes rules on international travel, ministers have been warned.

New rules allowing fully vaccinated passengers from the US and amber-list European countries to avoid self-isolation on arrival in the UK came into force at 4am on Monday morning.

But despite the relaxation of quarantine requirements, uncertainty continues around international travel, with Boris Johnson being warned not to create a new category in the traffic light system.

The rumoured amber watchlist would apply to countries at risk of being moved into the red category – which requires hotel quarantine for 10 days at a cost of £1,750 for an adult.

Reports have suggested there is a fallout among Cabinet ministers over proposals to introduce an amber watchlist, designed to warn travellers of the possibility that countries with concerning coronavirus data – which could potentially include Italy and Spain – could be put onto the red list at short notice.

Travel expert Paul Charles has said it would be a “disaster” if the Government introduced a new travel traffic light category.

The chief executive of travel consultancy The PC Agency told ITV’s Good Morning Britain programme: “It would be a disaster to bring in an amber watchlist on top of the amber list, the green list, the red list.”

As well as the green, amber and red categories, there is also “amber plus” – currently only containing France, which means passengers returning from the country’s popular holiday destinations are still required to self-isolate.

Tim Alderslade, chief executive of trade body Airlines UK, told Times Radio the decision on France meant “people now think with amber, there’s a good chance that whether there’s a watchlist or not, that they will be stranded, and that is a real dampener in terms of bookings”.

“We’ve now only got six to eight weeks until the end of the summer, and tens of thousands of jobs are under threat in the travel and aviation sector.”

Huw Merriman, the Tory chairman of the Commons Transport Committee, told the BBC: “An amber watchlist will be viewed as a massive red flag which is likely to cause bookings to those countries on that watchlist to collapse.

“In my view we don’t need any more uncertainty, complexity or anxiety for passengers or this beleaguered sector, it just needs clarity.”

Government minister Matt Warman defended the idea of a travel watchlists.

The digital infrastructure minister told Sky News said it allowed people to make “informed decisions” about the “direction of travel that a country is going in”.

“People do have to make common sense judgments and that may involve taking into consideration the fact that a country’s rates may indeed be getting worse,” he said.

“The most important thing that the Government can do is make sure that people have as much information as they possibly can, that they have information about which direction a foreign country might be going in so that they don’t inadvertently find themselves having to quarantine when they get back.”

Ministers are due to meet on Thursday to review the traffic light system and decide the travel rules that will be in place for most of August.

The Daily Telegraph reported that Transport Secretary Grant Shapps is pushing for the “amber watchlist” idea to be shelved, while Chancellor Rishi Sunak reportedly told the Prime Minister that the UK’s entry and exit rules are “out of step with our international competitors” and are hurting the economy.

The relaxation of rules on Monday allowed passengers who have been double-jabbed with a vaccine approved by regulators in the US, the EU or Switzerland to avoid 10 days in self-isolation,

They will be required to take a pre-departure test, and a PCR test on or before the second day after they arrive in England.

Heathrow chief executive John Holland-Kaye told BBC Radio 4’s Today: “We need to get rid of the requirement for everyone to have a PCR test and replace it with a cheaper, simpler lateral flow test and only use the PCR test if people test positive, for genomic purposes.”

Labour Party chairwoman Anneliese Dodds warned against creating “additional confusion and chaos” through an amber watchlist.

“The problem is, right now holidaymakers just don’t know who to believe,” she told Times Radio,” adding that we “seem to have the Chancellor briefing against the Prime Minister in the Sunday papers”.

“That’s not building confidence, ultimately, in the system.”