Boris Johnson said he wanted travel rules to be as simple as possible as he faced a backlash over the possible introduction of an “amber watchlist” of countries.

The Prime Minister faces Tory unrest, concern within the aviation industry and international criticism over the approach to international travel.

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.

Despite the relaxation of quarantine requirements, uncertainty continues around international travel, with the possibility of the new watchlist.

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.

Asked if he personally backed such a move, Mr Johnson said: “I understand how much people plan, prepare, for the summer holidays.

“But we have also got to remember this is still a dangerous virus and we must try and stop variants coming in, must stop importing variants from abroad, so we have to have a balanced approach.

“What I want to see is something that is as simple and as user-friendly for people as possible.”

The Prime Minister said the success of the vaccination campaign meant the British economy was “just about the most open in Europe” but there was a balance to be struck on travel because of the risk of importing new coronavirus variants.

“We also have to recognise that people want, badly, to go on their summer holidays, we need to get the travel industry moving again, we need to get our city centres open again and so we want an approach that is as simple as we can possibly make it.”

Travel expert Paul Charles, chief executive of The PC Agency consultancy firm, 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 are still required to self-isolate despite the relaxation of rules for journeys from elsewhere in Europe.

That status has angered the French government, with Europe minister Clement Beaune demanding an urgent review because he could see “no scientific or health reasons” for the situation.

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”.

He added: “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, 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.”

Fellow Tory MP Henry Smith, chairman of the all-party parliamentary group on the future of aviation, told the PA news agency the Government risked “squandering the successful vaccine rollout” with excessive restrictions.

“We are becoming increasingly at a competitive disadvantage to many other countries who have opened up international travel far more, many European countries and the US in particular,” he said.

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.”

In other developments:

– The NHS Covid-19 app is being modified to reduce the number of people being told to self-isolate, with close contacts now being “pinged” if they have been near a positive but asymptomatic individual in the two days before their test, rather than five.

– Some 46,872,411 people have received a first dose of vaccine, a rise of 21,266 on the previous day, while 38,464,025 are double-jabbed, an increase of 118,184.

– As of 9am on Monday, there had been a further 21,952 lab-confirmed Covid-19 cases in the UK.

– A further 24 people had died within 28 days of testing positive for Covid-19 as of Sunday, bringing the UK total by that measure to 129,743.