Jet2 has extended its suspension of flights and holidays until June 23 over a "lack of clarity" from the government over how and when international travel can resume.
A framework for the restart of overseas travel was set out on Thursday evening, but government officials refused to say if foreign holidays will be able to go ahead from May 17, as set out in the lockdown roadmap.
It was announced there would be a traffic light system with varying restrictions for destinations in each category, but there was also no suggestion as to which countries would be subject to what rules.
Jet2 chief executive Steve Heapy said his firm had studied the Global Travel Taskforce's framework and is "extremely disappointed at the lack of clarity and detail".
"After several weeks exploring how to restart international travel, with substantial assistance and input from the industry, the framework lacks any rigorous detail about how to get international travel going again. In fact, the framework is virtually the same as six months ago.
"Following the publication of the framework today, we still do not know when we can start to fly, where we can fly to and the availability and cost of testing. Rather than answering questions, the framework leaves everyone asking more."
He added: "Because of the continued uncertainty that the framework provides, it is with a heavy heart that we have taken the decision to extend the suspension of flights and holidays up to and including 23rd June 2021."
Jet2 is the latest airline to express discontent with the government's plans, with a number of others slamming the way travellers will be forced to take expensive coronavirus tests when leaving and entering the UK, no matter where they fly.
EasyJet chief executive Johan Lundgren claimed the plan to charge travellers £120 per person for tests upon arrival to the UK was “a blow to all travellers” and risked “making flying only for the wealthy”.
He added: “As the rest of British society and the economy opens up, it makes no sense to treat travel, particularly to low-risk countries, differently.”
Even Transport Secretary Grant Shapps has admitted the polymerase chain reaction (PCR) tests travellers must take are too expensive.
Mark Tanzer, boss of travel trade organisation Abta, said permitting the use of cheaper lateral flow tests would “make international travel more accessible and affordable whilst still providing an effective mitigation against reimportation of the virus”.
Tim Alderslade, chief executive of Airlines UK, the industry body representing UK-registered carriers, said the announcement “does not represent a reopening of travel as promised by ministers”.
The Department for Transport (DfT) said the government will work with the travel industry and private testing providers to reduce the cost of foreign trips, which could potentially involve free pre-departure tests and cheaper tests for when holidaymakers return.
It comes three days after Boris Johnson pledged to make testing requirements “as affordable as possible”.
Plans set out for the restart of international travel include a traffic light system to determine if and how long people will need to isolate for upon arrival back into the UK.
But Transport Secretary Grant Shapps was unable to say which countries will be placed into the "green zone", which will have much more relaxed restrictions than amber and red.
These are the rules for each category:
Green: There is no need to self-isolate. Take a pre-departure test and a PCR test on day two of your arrival in the UK.
Amber: Self-isolate for 10 days, unless you receive a negative result from a test taken at least five days after arrival. Take a pre-departure test, and PCR tests on day two and day eight of your arrival in the UK.
Red: Spend 11 days in a quarantine hotel. Take a pre-departure test, and PCR tests on day two and day eight of your arrival in the UK.
Assessments will be based on a range of factors, including the proportion of a country’s population which has been vaccinated, rates of infection, emerging new variants and the country’s access to reliable scientific data and genomic sequencing.
The categorisation of countries will be “kept under review” with a “particular focus on variants of concern”, the Department for Transport said.
Asked which countries might be in designated 'green', the transport secretary told ITV's Good Morning Britain: "At the moment, we don't know and the reason for that is with coronavirus things move very quickly."
He said at "the beginning of May" the Joint Bio-security Centre will make recommendations for which category a country should be in "so we'll be able to give people a bit more notice".
He also admitted to that the cost per person for the necessary tests to fly is "too high".
Asked if people could start to book foreign holidays now, Mr Shapps told Sky News: “I’m not telling people that they shouldn’t book summer holidays now, it’s the first time that I’ve been able to say that for many months.
“But I think everybody doing it understands there are risks with coronavirus and of course actually, I think people would want to be clear about which countries are going to be in the different traffic light system.
“So there is only two or three weeks to wait before we publish that list itself. But yes, tentative progress, for the first time, people can start to think about visiting loved ones abroad, or perhaps a summer holiday.
“But we’re doing it very, very cautiously, because we don’t want to see any return of coronavirus in this country.”
While more detail was set out, the DfT refused to say which countries Brits may be able to travel to, saying: “It is too early to predict which countries will be on which list over the summer, and the government continues to consider a range of factors to inform the restrictions placed on them.
“We will set out by early May which countries will fall into which category, as well as confirming whether international travel can resume from 17 May.”
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Restrictions will be “formally reviewed” on June 28 to take account of “the domestic and international health picture and to see whether current measures could be rolled back”, the department added.
Further reviews will take place no later than July 31 and October 1.
A “Green Watchlist” will be introduced to identify countries most at risk of moving from “green” to “amber”.
Paul Charles, chief executive of travel consultancy The PC Agency, said the government is “making the right moves to help the sector restart successfully in May” but there are “still too many layers of complexity”.
The government announced plans to digitise the Passenger Locator Form to enable checks to take place at e-gates by autumn 2021.
It also revealed the Civil Aviation Authority will be given additional enforcement powers to act on airlines that breach consumer rights, after many passengers struggled to obtain refunds when flights were grounded.