The government has urged Brits not to book foreign summer holidays yet despite lockdown measures easing as Boris Johnson warned of the risk of re-importing Covid-19.

The prime minister told a news conference he was “hopeful” that non-essential international travel would restart from May 17, but that he did not want to underestimate the growing number of Covid-19 cases elsewhere.

Britain plans to use a traffic-light risk system for countries once international travel resumes, but the government said it was too early to say which countries could be given the green light that would only require coronavirus tests before and after travel.

A government review published on Monday said: “Taking into account the latest situation with (coronavirus) variants and the evidence about the efficacy of vaccines against them, we will confirm in advance whether non-essential international travel can resume on 17 May, or whether we will need to wait longer before lifting the outbound travel restriction.”

It added: “The government advises people not to book summer holidays abroad until  the picture is clearer.”

At the Downing Street briefing, Johnson said: “Obviously we are hopeful that we can get going from May 17, but I do not wish to give hostages to fortune or to underestimate the difficulties that we are seeing in some of the destination countries that people might want to go to.

“We don’t want to see the virus re-imported from another country. Plainly there is a surge in other parts of the country, and we have to be mindful of that.”

Reports have suggested countries on the green list, requiring testing before and after travel, could include Portugal, Malta, Israel, the United Arab Emirates and the US.

Under Johnson’s original plan, international travel would not resume until May 17 at the earliest. Countries on the amber list would require self-isolation. Those on the red list would require quarantine.

Airlines such as easyJet, Ryanair and British Airways, plus holiday groups such as TUI, hope to avoid a second lost summer but Covid-19 cases have risen in continental Europe.

Johnson said a planned reopening of the economy could take place next week, with the opening of all shops, gyms, hairdressers and outdoor hospitality areas in England.

With the vaccine programme rolling out rapidly across Britain and infection numbers falling, Johnson said England could proceed to Stage 2 of his roadmap out of lockdown from April 12.

“On Monday the 12th, I will be going to the pub myself – and cautiously but irreversibly raising a pint of beer to my lips,” Johnson said.

Britain said people should continue to work from home where they can and minimise domestic travel.

Johnson also confirmed that the government was looking at a Covid-19 status certification system, or vaccine passport, to help reopen larger events and to travel.

“I want to stress that there are complicated, ethical, and practical issues… raised by the idea of Covid status certification… using vaccination alone,” Johnson said.

“You’ve got to be very careful in how you handle this and … don’t start a system that’s discriminatory.”

People will not need vaccine certification for pubs, hairdressers and shops, Johnson said.