Airlines have criticised the British government's new traffic light system for travel, saying the cost of PCR testing for those arriving from low-risk countries will put off many travellers.
Britain will confirm early next month whether it will allow international travel to resume from 17 May and which countries will fall into the red, amber or green categories in a traffic light system based on Covid-19 risks.
Airlines, desperate for a bumper summer after a year of restrictions, criticised proposals from the government's Global Travel Taskforce, which include Covid tests, saying the £100 cost of PCR testing for those arriving from low-risk green countries would put off many travellers.
Giving new details of how it hopes to allow people to travel this summer, the taskforce also said work was going on to develop a certification system, sometimes called "vaccine passports", for inbound and outbound travel.
Transport minster Grant Schapps said the public can now think about booking summer holidays abroad but there were still risks.
"I'm not telling people that they shouldn't book some holidays now. It's the first time I've been able to say that for many months. I think everybody doing it understands there are risks with coronavirus," he told Sky News.
"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," Mr Schapps added.
Jet2.com announced it was extending the suspension of its flights and holidays up to 23 June following the government's announcement.
"We have taken time to study the Global Travel Taskforce's framework, and we are extremely disappointed at the lack of clarity and detail," chief executive Steve Heapy said.
"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," Mr Heapy said.
"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."
"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 23 June 2021," he added.
Airlines UK, an industry body which represents British Airways, easyJet, Ryanair, Virgin Atlantic and others said the new system "does not represent a reopening of travel as promised by ministers."
EasyJet, Britain's biggest airline by passenger numbers, said the PCR test requirement for low-risk countries was a blow to travellers and called on the government to re-assess its plan.
"This risks reversing the clock and making flying only for the wealthy," said easyJet chief executive Johan Lundgren.
The chief executive of Virgin Atlantic has also criticised the UK government's proposals for restarting travel outlined, saying that trips to low-risk countries should not involve expensive PCR tests for Covid-19.
"For travel between green countries it should be absent quarantine and absent testing," chief executive Shai Weiss said. "There are better ways of doing what the government has set out to do."
He said the more expensive PCR tests required for travel would put off some customers.
Case numbers in Britain have dropped dramatically since the January peak, but a government priority is to avoid undermining the success of its vaccination programme by importing vaccine-resistant variants from overseas.
The government said it was working with the travel industry and with private Covid-19 test providers to cut the cost of travel.
"This could include cheaper tests being used when holidaymakers return home, as well as whether the government would be able to provide pre-departure tests," the travel taskforce said.
Under the traffic light system, restrictions such as hotel quarantine, home quarantine and compulsory Covid tests will apply differently depending on which category of country a passenger arrives from.
Factors in assessing categories will include the percentage of the population that has been vaccinated, the rate of infection, the prevalence of variants of concern and the country's access to reliable genomic sequencing.
There will be a "green watchlist" identifying countries most at risk of moving from green to amber, although the government said it would not hesitate to change a country's category should data show risk had increased.
The taskforce indicated that a digital travel certification system would be part of the plan but gave few details beyond saying that Britain wanted to play a leading role in developing standards in this area.