A problem with the NHS app prevented travellers from accessing their Covid passports and caused some to miss flights on Wednesday.
At around 1pm, NHS Digital said it was investigating a technical difficulty stopping people from loading their passes.
The app allows individuals to access and download a pdf proving that they have had one or both Covid vaccinations, which needs to be shown when travelling abroad. NHS Digital said the app would be back up and running by around 4pm on Wednesday.
But travellers took to social media to complain that the glitch had caused them to miss flights.
Chuck Adolphy said his "dream holiday" had descended into a "shambles" on Wednesday morning when he and his girlfriend attempted to board a flight from Gatwick to Ljubljana, the Slovenian capital, before being turned away.
"Went to go on the app and my Covid Pass wasn’t appearing," he said. "Didn’t even realise at the time there was an outage and was just turned away and told they couldn’t do anything about it.
"Rang the number and they said the app and site are down and there is nothing they can do. EasyJet said nothing either. Showed them my vaccine card and nothing."
The NHS app has gained more than 10 million new users in recent months after it became the platform for accessing Covid passes, which show proof of vaccination status and is required for international travel.
The passes can also be issued in paper form, as PDFs or to be added to smartphone wallets, but this would have had to have been done by passengers before the app problem struck.