The feature, which details the vaccine doses a person has received, is needed by those wishing to travel abroad as proof of inoculation.
Users reported seeing an error message on Wednesday afternoon on the NHS smartphone app, saying that high demand was “limiting access to the service”.
Responding to the problem, NHS digital said in a statement: “There are currently issues with accessing the Covid Pass on the NHS App and website. We are investigating the issue and will update as soon as we can.”
Holidaymakers complained about the outage on Twitter, with several people claiming they had missed their flights because of the issue.
Henry Martin, who works in digital communications, tweeted that he was unable to board a flight home without the certificate. “This has been an absolute nightmare. We’re stuck here in Greece as they wouldn’t let us on plane without a pass,” he wrote.
Another traveller, who is scheduled to fly to the Netherlands on Thursday, said the app error risked jeopardising her holiday. “This is disgraceful. We have complied with every restriction and requirement asked of us and still we might not be able to travel,” Pam Whittingham said.
However, NHS Digital told The Independent that the issue was resolved at 3.15pm on Wednesday afternoon, three and a half hours after the error first occurred.
The Covid Pass is also required at some events in England, although most entertainment venues do not require it.
The vaccine certificate is also available as a paper version but takes up to five working days to arrive, according to the NHS.