The National Health Service (NHS) reported problems with user access to the Covid Pass on the NHS app and the website, adding that the issue is being investigated.
The NHS Covid Pass is used to prove one's vaccination record and allows people to travel, as well as access events and venues in the UK and other countries.
Travellers took to social media on Wednesday to complain and fume about the incident, saying they couldn't take their trips, take tours or go to museums.
The NHS said it would update users on the matter "as soon as we can."