The NHS Covid Pass experienced technical difficulties on Wednesday hours after the Prime Minister announced a move to Plan B in England.
Users reported on social media being unable to download their domestic or travel passes from the NHS England app.
Those who tried to download the domestic pass were faced with a loading screen before it diverted to a page outlining how to get the pass.
Others wanting to download their travel pass were presented with a message which said: “An unexpected error has occurred.”
Some users said they got a message which said: “Please try again later. 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.”
It comes hours after Boris Johnson announced England would be moving into Plan B following a spike in cases of the Omicron variant of coronavirus.
The NHS Covid Pass, which can be obtained by having two vaccines or a negative lateral flow test, will be introduced for entry into nightclubs and other large venues from December 15.
The passes can be downloaded from the NHS app and saved onto mobile phones or saved as a PDF and printed off. They last for 30 days.
A statement from NHS Digital said: “We are aware of an issue affecting access to the NHS Covid Pass on the NHS App and website. We are investigating this as a priority and will update as soon as we can.”
A further statement issued later added: “We are continuing to investigate the current issues with the NHS COVID Pass and will provide an update as soon as possible. We apologise for any issues this may have caused and appreciate your patience as we work to resolve it.”