Scotland's newly launched vaccine passport app has been hit by technical problems meaning people are unable to register their details on it.
The NHS Scotland COVID Status app was made available to download on Apple and Android devices on Thursday afternoon.
From 5am on Friday, people attending large events and nightclubs will need to show proof using the app that they have had two doses of vaccine before they are allowed in.
But just hours after the app's launch, comments circulating on social media suggested many users have been unable to register on it.
The Scottish government said the large volume of people accessing the app at once could be a reason behind the glitch.
One Twitter user posted: "Didn't matter if I had my NHS number or not, 'no match found' is all I get."
Others shared screenshots showing they were unable to get past the first log-in page.
A Scottish government spokesperson said: "Some people may be experiencing issues which are likely to be caused by extremely high initial traffic and a large number of users trying to access the app at once.
"We advise people whose data is not found to try again a couple of hours later."
The launch came on the same day the Night Time Industries Association (NTIA) Scotland lost a legal bid to delay the rollout of vaccine passports.
Following consultation with businesses affected, the Scottish government confirmed a grace period has been agreed which means the scheme will not be legally enforced until 18 October.
The legal requirement for businesses to keep information about certification status confidential and not use it for other purposes came into effect on Friday.
Speaking about the app's launch, Deputy First Minister John Swinney said: "We know from expert public health analysis that we must do all we can to stem the rise in cases and reduce the pressure on the NHS.
"Vaccine certificates have a role to play as part of a wider package of measures. They add a further layer of protection in certain higher risk settings.
"This is a very limited scheme and we hope this will allow businesses to remain open and prevent any further restrictions as we head into autumn and winter.
"I would encourage people to download the NHS Scotland COVID Status app and help our drive to keep the virus under control.
"Scotland is not unique in introducing such an approach. Certification schemes are in place across Europe, including Ireland and France, and the Welsh government is also planning similar measures.
"I also want to ensure that as many people get vaccinated as possible and particularly to increase uptake in the younger age cohort, so anything that helps to incentivise that is helpful."
There are currently no plans to introduce certification for the wider hospitality industry but this will be kept under review over the autumn and winter months, according to the Scottish government.
Exemptions to the scheme include the under-18s, participants in vaccine trials, as well as people who cannot be vaccinated for medical reasons and people working or performing in the venues."