The Prime Minister’s spokesman said he was “confident” the AstraZeneca jab would be recognised by US border officials despite the vaccine not being approved by the US Food and Drug Administration (FDA).
It comes as Downing Street was forced to deny that Boris Johnson was left in the dark about Washington’s announcement that it would be reopening its borders in six weeks.
The White House will lift the 18-month blanket ban on foreign travellers from entering the country, which was introduced by former president Donald Trump at the start of the coronavirus pandemic.
Mr Johnson’s spokesman, asked by reporters in New York whether there had been reassurances from US officials that the AstraZeneca jab would be recognised to allow British travellers to enter, said: “I have got no indications that it won’t be.
“I am confident that every vaccine we have used, any vaccine received in the UK and approved by the NHS, obviously signed off by the MHRA (Medicines and Healthcare products Regulatory Agency), WHO (World Health Organisation) will be applicable.”
The FDA recognises people who have received the Pfizer, Moderna or Janssen from Johnson & Johnson jab as fully vaccinated, but travellers who have received the AstraZeneca vaccine will also be able to enter because the US acknowledges its approval by the WHO.
US chief medical adviser Dr Anthony Fauci told BBC Radio 4's Today programme: "I don't believe there's any reason to believe that people who have received the AZ vaccine should feel that there is going to be any problem with them."
He was asked if he expects that anyone who has had a vaccine approved by the UK Government will be able to travel to the US.
Dr Fauci told the programme: "I can't account for every vaccine that has been approved by the UK.
"I am not sure about all of them but the specific one about AZ, given that we have a substantial amount of information on the AZ vaccine - again without being definitive about it - I would predict that there would not be a problem there.
"The final decision goes with the CDC."
They have repeatedly blamed the travel ban for limiting the recovery of passenger numbers during the virus crisis.
Heathrow has gone from being Europe’s busiest airport in 2019 to 10th, behind rivals in cities such as Amsterdam, Paris and Frankfurt.
The Prime Minister said he was “delighted” that US President Joe Biden was “reinstating transatlantic travel” but later faced questions about whether he had been kept out in the cold regarding the decision.
Speaking on the plane from London to America on Sunday, Mr Johnson had previously told reporters: “I don’t think we’re necessarily going to crack it this week.”
At a press briefing on Monday in New York, the Conservative Party leader said it was “thanks to the hard work of our teams” that the announcement had come “faster than we expected”.
Downing Street, in a later briefing with reporters, denied any suggestion the UK leader had been “blindsided”.
Mr Johnson’s spokesman said: “We were aware an announcement was pending.
“We were kept in the loop.”
No 10 said Britain represented a “significant market” to the States and that a US-UK task force, agreed at the G7 leaders’ meeting in Cornwall in June with the aim of opening up travel, had “helped expedite” Washington’s decision to relax its border controls.
Around 3.8 million British nationals visited the US every year prior to the pandemic, according to the Foreign, Commonwealth and Development Office.
White House Covid-19 co-ordinator Jeff Zients, who announced the end of the travel ban, said all foreign visitors will need to demonstrate proof of vaccination as well as proof of a negative test taken with the previous three days.
Airlines will be required to collect contact information from international travellers so that they can be traced if required.
The move to open US borders was heralded by the bosses of BA, Virgin Atlantic and Heathrow.