Britons are being urged not to share pictures of their negative lateral flow tests online amid fears the codes will be used by criminals to sell fake results.
Reports have already suggested a growth in the sale of illegal COVID vaccination certificates across Europe as more countries require them for travel and access to events.
There are now concerns that gangs are using codes from tests posted on social media to sell negative results to help people get into events.
Shahzad Ali, CEO at security training company Get Licensed, said venues like nightclubs have had to deal with fake documentation for years and COVID tests could be the latest market for criminals.
He told Wales Online: "We have seen fake documentation for many years, for example, fake IDs have been a regular feature at nightclubs for a number of years.
"This is just a new complication that door supervisors will soon become used to facing. It was always inevitable that fake COVID passes would start to appear as soon as there were rumours of them being introduced."
Watch: How do COVID passes work in the UK and France?
He warned that people can face fines of up to £10,000 if they are caught using, supplying or distributing fake COVID passes.
He added: "There is obviously going to be a market for COVID passes, because there will be people who want to go about their life like normal and not have to take COVID tests for things they didn’t have to before, so it is extremely important that you look after your COVID pass.
"Our advice would be to avoid posting it on social media, don’t share the code from the lateral flow you have taken because others could register it as their test.
"COVID passes will potentially make door supervisors' jobs much more difficult, especially when we consider the consequences of people who are especially forthcoming with their beliefs, it could make the job much more dangerous."
NHS COVID passes were introduced in some settings in England as part of Plan B measures to limit the spread of the Omicron variant of coronavirus, while proof of a negative lateral flow test is also required in some situations.
Earlier this month it was reported that fraudsters were directing people on Facebook to sites claiming to sell fake COVID vaccine passes for people who have not been vaccinated.
Jonathan Benton, a former senior detective who runs cyber investigation firm Intelligent Sanctuary, told the BBC that COVID certificates were found for sale on the dark web alongside drugs, weapons and stolen goods.
Watch: What is a Rapid Lateral Flow Test?