Travellers were left unable to board flights on Wednesday after the Covid Pass within the NHS App failed to load.
The issue resulted in air passengers being turned away, while Britons already on holiday were banned from entering tourist attractions.
People travelling abroad must show a QR code within the app to prove their vaccine status or print out a letter in advance.
By around 3.15pm, the service was working again. NHS Digital said the glitch was caused by a “technical issue with a global service provider”.
But it was too late for some holidaymakers, including Chuck Adolphy and his girlfriend, who were not allowed to proceed for an EasyJet flight from London Gatwick to Slovenia’s capital Ljubljana.
Speaking to the PA news agency, he described the incident as a “shambles”.
“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 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.”
Callum Melia from Liverpool also told PA he was unable to access tourist attractions on his holiday in Naples, Italy, as a result of the app issue.
“We’re about to enter these tours around the city when we discover we can’t get on the app to access the QR code,” the 26-year-old told PA.
“We also had further problems accessing bars and restaurants.”
The NHS App has gained more than 10 million new users in recent months as it became the platform for accessing the pass, which shows proof of a person’s vaccination status and is required for international travel.
A spokesperson for NHS Digital said: “The NHS Covid Pass service was temporarily unavailable between 11:45 and 15:15 today as a result of a technical issue with a global service provider that affected many different organisations.
“Following an urgent investigation, this has been fixed and Covid Passes are available both via the NHS App and online.”