British travellers were left struggling to board flights after the NHS COVID pass became temporarily unavailable.
Users trying to access proof of their vaccination status via the NHS App and website found that the service was inaccessible from about 3.50pm on Thursday.
"We are sorry the NHS COVID pass is currently unavailable. At this time there is no alternative route for accessing this information via the NHS App or online," an error message read.
"If you have already downloaded the information, then your access should not be affected."
On social media, several travellers said they had been unable to check in at airports because they were unable to prove their vaccination status.
The requirement for people to show the NHS COVID pass to visit indoor and outdoor events and venues in the UK was scrapped earlier this year.
But, it is still used as a way for people to show their vaccination status when travelling abroad to certain countries, such as Spain.
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'I've got a flight at 6am'
Jon May was set to fly to Valencia from Bristol early on Friday morning for a work conference - but when he tried to download his proof of vaccination, he found that it was down.
The 32-year-old said: "I've had all three jabs, and I went to the NHS App... and it said 'service unavailable'. I was like, 'what do you mean, service unavailable? I've got a flight at 6am'."
Mr May, who works in marketing, said he did not have any other documentation to prove he's been vaccinated and he was nervous about being detained at the Spanish border.
"If it doesn't work, I'm hoping they just go with it... I have had all my vaccinations, I just can't get the certification," he said.
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Fi Bartlett, 24, from Bournemouth, was unable to complete her online check-in for her flight to Malaga on Saturday without proof of vaccination and said she had planned to go to the airport with her paper vaccination confirmation if it had not been fixed in time.
NHS Digital said it was aware of the issues and was "urgently" investigating the matter.
The service resumed working at 10pm after the technical issue was resolved.
