The government has admitted the NHS app might not be able to be used as a vaccine passport when international travel is due to resume on May 17.
Brits wanting to travel to foreign countries will be forced to show evidence they're vaccinated, have tested negative for coronavirus or have coronavirus antibodies.
Transport Secretary Grant Shapps has previously said the NHS app, which is currently used to book medical appointments and order repeat prescriptions, will be able to display evidence that someone in England has been vaccinated or tested.
But the Prime Minister's official spokesman indicated that officials are working on alternative plans for when international travel resumes, which is expected on May 17 for people in England.
The spokesman said: "Mr Shapps set out the approach we are looking to take.
"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."
The spokesman added: "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."
Henk van Klaveren, head of public affairs at trade body The Airport Operators Association, said enabling the app to prove a user's vaccine status "should be a relatively simple technical step" but he is "not as confident" about whether it will integrate test results.
He told the All-Party Parliamentary Group (APPG) on Coronavirus it "has to be the case" that the UK has a "four-nation approach" to the issue.
He said: "From Wales, Northern Ireland and Scotland there is currently no road map on how international travel will restart.
"And similarly, there is no approach at the moment on a four-nation basis for a vaccine passport."
Premier League executive director Bill Bush told the hearing that "we do support some form of certification for our events", suggesting that the alternative was restrictions in the form of social distancing and smaller crowds.
He described a form of certification, provided it was voluntary, as "burdensome" but an "acceptable" way to allow fans to attend matches.
