Hopes are rapidly fading that the US and UK will agree to open an air corridor before the end of the summer, in the latest sign that a rise in coronavirus cases in Britain is hobbling millions of people’s travel plans.

Officials involved in talks about a US-UK travel corridor, which started last week, said they thought it was increasingly unlikely that they would reach a conclusion by the end of next month, as some had originally expected.

Instead, they said a combination of the rise in cases of the Delta variant in the UK, the complexities of the US political system and uncertainty over the status of AstraZeneca’s vaccine were set to extend the talks into August and even September.

The predicted delay in agreeing the air corridor was the latest in a string of difficulties racking up for Britons planning to travel abroad this summer.

On Monday, Spain, Portugal, Malta and Hong Kong announced tighter restrictions on travellers entering from the UK.

Officials in London had hoped they would have the outline of an agreement to reopen US-UK travel by US Independence Day celebrations on July 4. But British government insiders believe that is likely to prove impossible.

One UK official briefed on the talks said: “This is not going to happen soon. We thought July was the earliest we might be able to get something in place, but now it’s looking more like September.”

Another person familiar with the discussions said the UK was pushing for an agreement far more than the US.

“The Biden administration is in no hurry . . . and the chances of anything happening before August now seem to be zilch,” added this person.

Non-Americans have been barred from travelling from the UK to the US since the start of the Covid-19 pandemic last March unless they are a green card holder, the immediate family of a US citizen or apply for a special exemption.

The ban was put in place by then-US president Donald Trump, and was maintained by his successor Joe Biden. The UK allows anyone to enter from the US, although they have to quarantine for at least five days.

British officials have struggled to persuade their US counterparts to drop the restrictions. They had hoped to make headway after Biden this month agreed to set up a working group of US and UK officials to discuss how to restart travel.

The UK on Monday reported 22,868 new coronavirus cases in the latest 24 hour period — a level last recorded in January. But with more than 60 per cent of the adult population fully vaccinated, deaths have remained low.

Finalising a US-UK travel corridor has been bedevilled by the number of US government departments that have a say on coronavirus-related travel rules, including the transport department, the Centers for Disease Control and Prevention, the state department and the White House.

The status of the Oxford/AstraZeneca Covid-19 vaccine in the US further complicates matters. The Anglo-Swedish pharmaceutical company has not yet applied for authorisation from the US Food and Drug Administration, and when it does so, it is likely to apply for a full legal licence rather than temporary emergency authorisation — a process that could take several months to complete.

US officials have not said whether they intend to treat vaccinated and unvaccinated travellers differently.

But UK diplomats fear that if they do, Britons who have had AstraZeneca’s jab could face tougher restrictions than those who have received US-approved vaccines.

One UK diplomat said: “AstraZeneca is proving a real problem. If the US doesn’t recognise it, it means millions of Brits won’t be eligible to travel if we agree to a new corridor.”

The White House said meetings about a US-UK travel corridor were “active and ongoing”.

The British government spokesperson said the US-UK working group was established “to help relaunch UK-US travel as soon as possible”.

“Discussions between the working group are ongoing to ensure the UK and US closely share thinking and expertise on international travel policy going forward,” added the spokesperson. AstraZeneca declined to comment.

Additional reporting by Jim Pickard, Donato Paolo Mancini and Philip Georgiadis