The NHS Covid Pass, used to show a person’s vaccine status for travel and events, stopped working on Wednesday.
The feature, which is available in the NHS smartphone app, typically allows users to access a barcode or text records detailing which vaccine doses they have received.
Instead, users receive error messages or notices that high traffic volumes are “limiting access to the service.”
NHS Digital said in a statement that it was looking into the problem.
“At the moment, there are issues with accessing the Covid Pass on the NHS App and website,” it said, promising an update “as soon as we can.”
Subscriptions and other features of the general-purpose NHS app appeared to be operational.
Many of the replies to a tweet from NHS Digital were from worried travellers who were not sure they could prove their vaccinated status at airports or other departure points without it – including one person who claimed they missed their flight because of the problem.
The Covid Pass can be saved as an offline version that is valid for 30 days. Some users simply take a screenshot of their barcode, and iPhone users can save a copy to their Apple Wallet.
But those methods needed to have been done before the system went down.
The Covid Pass is also required for entry to some events and locations in England, though this is becoming less common as restrictions at most entertainment venues have been relaxed.
For those over the age of 16, who were vaccinated in England, Wales, or the Isle of Man, a digital version is now available.
A paper version of the pass is also available as a letter for fully-vaccinated people in England. The letter is sent to the address on file with a patient’s local GP surgery, but can take up to five working days to arrive, the NHS says.
The vaccine record in the NHS app is distinct from the NHS Covid-19 app, which was widely used earlier in the pandemic to estimate exposure to people who tested positive for the virus and “pinged” many people with self-isolation alerts.