Holidaymakers may have to use paper Covid certificates to prove they have been vaccinated when the foreign travel ban is lifted from May 17, it emerged on Tuesday.
Downing Street admitted that the NHS app may not be ready for use as a vaccine passport in time and the Government is preparing for "another approach" for holidaymakers travelling abroad in the interim.
At least 20 countries have already indicated that they will accept vaccination certificates as an alternative to PCR tests for holidaymakers to prove their Covid status. Only Greece, however, has so far explicitly said that it will take paper certificates, despite concerns that they are more easily forged.
Rita Marques, Portugal's tourism minister, has said her country would accept only a digital pass because of the risk of fakes. Spain, due to trial its app at more than 40 airports, has said it is in talks with the UK Government over the need for a "mutually recognisable" app.
A senior industry source involved in discussions on the technology said: "I suspect they will take whatever evidence we have in the short term, whether it is another private app solution, a paper certificate, a letter from your GP or a negative PCR test."
It comes as the Government is expected to announce on Friday a "tiny handful" of "green list" countries to which holidaymakers can travel without quarantining on their return. Most of Europe will be "amber", requiring 10-day home quarantine and PCR tests when travellers return to the UK.
Without the acceptance of a vaccination certificate, travellers may instead have to prove their Covid status by taking a PCR or lateral flow test even if they are inoculated.
The NHS app, used for appointments and prescriptions, is being developed by NHSX, the health service's technology arm. The app allows patients to view their vaccination status if their GP has permitted access.
Grant Shapps, the Transport Secretary, said last week it would display evidence that someone has been vaccinated or recently tested.
But Boris Johnson's official spokesman indicated that officials were working on alternative plans for when international travel resumes, expected on May 17.
"Obviously we will be able to confirm ahead of the 17th at the earliest what measures are used for those initial countries that are available for travel, be it the app or another approach," said the spokesman.
"There are other routes to achieving the same end goal. We are working on the app at the moment, at pace, to have it ready, and we will be able to confirm ahead of the 17th at the earliest what approaches we will be using."
A Department of Health spokesman said: "The Government is working on providing individuals with the means to demonstrate their Covid status easily – through a digital route as well as a route for people who don't have access to a smart phone.
"Security and privacy will be at the core of our approach. Use of the NHS App is being considered as part of the digital route."
Paul Charles, the chief executive of travel consultancy The PC Agency, said: "Just because a country is on the UK's 'green list', it doesn't mean it won't have its own set of restrictions for when you enter. So people are going to have to be a bit of a sleuth to interpret which countries are truly open.
"Some may say we don’t need a vaccination passport because the UK has such a high level of inoculation. Others will insist on a PCR test like Britain even if you are vaccinated."