Private companies are listed on the site in order of price with the cheapest at the top. The lowest test priced at £23.
But analysis of the first 50 cheapest options found two thirds were not available to book at all or until at least after the peak holiday season.
Consultancy firm Fideres found the companies offering cheaper pay-for tests had the largest gap between advertised and real prices.
Paul Vella, an associate at Fideres, told The Guardian: “The Government is reporting prices that are a small fraction of what most consumers pay, without actually checking if these low prices are available.
“Not only does this mislead consumers, it can actually steer them into using more expensive providers.”
At the moment, unvaccinated amber list holidaymakers from England have to test before they travel and have booked two PCR tests for when they return to take during their 10-day quarantine.
Vaccinated travellers do not have to quarantine but still require a test two days after they return.
In Wales, they are free and supplied by the NHS.
By comparison the cheapest two-swab postal kit costs £78 in the UK, according to the research which was conducted by consultancy firm Fideres.
A spokesperson the Advertising Standards Authority said they would be investigating.
It told the Guardian: “We have received complaints about this, regarding inconsistent pricing, which we are currently reviewing.”
A Department of Health and Social Care spokesperson said: “We do not endorse or recommend any private Covid-19 test provider.
“All private providers must meet the minimum required standards and each of them are reviewed by the independent United Kingdom Accreditation Service.
“The government carefully monitors issues raised by the public and takes rapid action with companies where necessary, giving providers a five-day warning if their service is inadequate and if they do not rectify it, removing them from the appropriate travel test list.”