An advert for a coronavirus "passport" app, which has been promoted by Zara Tindall, has been referred to the advertising regulator over concerns it may be at odds with health guidance.
The VHealth Passport allows Covid test results to be uploaded to a phone - meaning people can demonstrate they are safe to attend sports and other events.
But in an ad to promote it, Mrs Tindall is shown having an antibody test.
Government guidance says these tests do not show if you currently have Covid.
The platform was initially referred to the Medicines and Healthcare products Regulatory Agency (MHRA), who concluded that it was not a matter for them and passed the issue to the Advertising Standards Authority (ASA) for further investigation.
The ASA confirmed that the matter had been referred to them.
VST Enterprises, which owns the app, said it had "fully engaged" with the MHRA over the concerns.
VST Enterprises said VHealth Passport was not being used as a so-called "immunity passport", adding that the platform "encourages the user to have a regular test cycle to manage their personal contact with the virus".
VHealth Passport "does not promote or recommend any specific Covid test or manufacturer", it said, and "will always be guided by medical science and the regulatory governing bodies" over tests.
The test kit used was for professional use only, CE marked and registered with the MHRA, VST Enterprises said.
Earlier, VST Enterprises said the particular antibody test featured in its video was used for "illustrative purposes".
After creating an account with VHealth Passport, members of the public can book a coronavirus test - with a medical centre that follows government guidelines - via the app.
The person's test results are then uploaded to VHealth Passport, which will display their current health status - as a unique digital code that can be scanned from more than 2m (6ft) away.
A negative test result will also show up as a green tick on the "passport".
Mrs Tindall, an Olympic medal-winning equestrian and the Queen's granddaughter, features in an advert for the app with her husband Mike, a member of England's 2003 Rugby World Cup winning squad.
In the video, she explains that the app can be used by sports men and women, as well as support staff, volunteers and fans, to give them confidence to attend large events again.
Mr Tindall has described the device as a potential "game-changer" for getting fans back to watching sports.
In the ad, viewers are told that Mrs Tindall is taking a test to see if she "currently" has the virus or whether she had previously had it. When her results are revealed, she is told she is "clearly negative" for Covid-19 "current" infection.
However, the video shows Mrs Tindall having an antibody test. Government guidance says such a test is limited to answering whether someone has previously been infected by Covid and understanding the spread of the virus.
It says antibody tests cannot tell you if you can or cannot spread the virus to other people.
The test taken by Mrs Tindall looks for three types of antibody - IgG, IgM and IgA. These start to appear a few days after coronavirus infection and stick around for weeks in the blood.
A negative result means there are no signs that someone has had coronavirus - but it doesn't give someone the all clear and mean they are "safe" in terms of infection risk.
They could still currently have Covid, and their body has not yet had time to make antibodies against it.
You would need to use a different test to tell if someone is currently infected and risks spreading it to others.
A positive antibody test result means that someone has, most likely, recently been infected and now has some immunity against the virus.
But, like any test, antibody checks can give false results.
Jon Deeks, a professor of biostatistics at the University of Birmingham, says he reported the advert to the MHRA.
"If people use these tests, they get a negative result and they go and think they haven't got Covid when they could actually still be infectious, that might mean that people don't social distance and so on," he said.
"And that might increase the risk of actually spreading Covid."
He has argued the tests would still miss cases in the very early stages of infection - when people are most likely to pass on the disease.
Simon Clarke, associate professor in the School of Biological Sciences at the University of Reading, said the government was right to say that antibody tests cannot test for current infection.
"They are a poor diagnostic and that is why they are not used," he said. "They are used to look to see who has had the infection in the past."
The BBC has approached Mr and Mrs Tindall for a comment.
VST Enterprises said it was correct that antibody tests are usually only used to identify previous infection.
But it added that the one it had featured was different - because it tested for antibodies that presented as early as three to four days after infection. That meant it was possible to use the test to screen for "possible current infection".
The platform has been submitted to the government as a way of getting fans safely back into sports stadiums, according to the firm.
In a statement, VST Enterprises said: "In this particular instance the VHealth Passport was demonstrating how a rapid Covid test kit could be used. It will work with any test that is recommended by government."
An MHRA spokesperson said: "Antibody tests are not used for diagnosis but rather to give a better understanding of the prevalence of the virus in different places and therefore cannot be relied upon for protection to COVID-19 or to or detect previous infection."
It comes as most sports in England have been played behind closed doors since lockdown was introduced in March. The chief executives of the Premier League and the Rugby Football League have urged the government to allow the return of crowds to games as soon as possible.
But in September, the government halted a plan for a phased return of spectators because of rising coronavirus case numbers.