There are still "serious problems" among private Covid testing firms, with rogue operators making misleading claims, says consumer group Which?.
Most travellers to the UK are required to pre-book tests for day two and day eight after their return.
But several test providers on a government approved list are offering misleading information about pricing and availability, Which? found.
The Department for Health said it was carefully monitoring test providers.
Problems with private test providers first came to light in April when there was an increase in the number of people travelling. Several firms, listed by the government as approved providers, failed to deliver test kits on time, or failed to return results.
Travellers were left quarantining at home unsure how to meet government testing requirements and in many cases unable to reach the testing companies online or by phone.
Some didn't receive test kits at all, despite having paid hundreds of pounds. A few received results for tests they hadn't taken.
Since 17 May the rules on international travel have been eased, and the vast majority of countries are on the government's "amber" list meaning travellers, including UK nationals, must pre-book a two-test package before they are allowed into the UK.
Now, Which? has found some providers are quoting headline prices for one test, when two are required. Others are marketing tests although they aren't ready to deliver them.
"Weeks on from some international travel being allowed to resume, it's very concerning to still be uncovering such serious problems with the government's testing system for travellers - problems that could have easily been ironed out well ahead of travel restarting, had proper regulatory oversight been ensured early on," said Rory Boland, travel editor at Which?
"As it stands, travellers risk being left at the mercy of rogue operators who, at best, attempt to profiteer off of those looking for testing services to allow them to travel, and at worst, risk leaving them out of pocket for services that don't even exist."
He said the government needed to sort out problems at testing firms before mass travel resumes, to avoid "chaos for travellers".
In late May, Which? investigated the three cheapest offers, that appeared on the list of approved providers from the Department of Health and Social Care (DHSC).
The prices offered by Biograd Diagnostics, Screen 4 and Book A Travel Test were between £60 and £98. However, when Which? investigated further it found the quoted prices were for a single test, not the compulsory two-test package.
The firms involved said the problem lay with how the DHSC recorded price information.
"A number of companies including ourselves were initially confused by the government listing forms," said a Book A Travel Test spokesperson.
"We have gone to great lengths to support travellers, however, we are not immune to costs and have had to navigate fluctuating charges from labs. We remain committed to doing all we can to deliver an excellent service to travellers during this difficult time.''
A Screen 4 spokesperson said the problem lay in "differences in terminology".
"We do not and have never tried to manipulate the listings and we have tried to provide a competitive and reliable service with clearly stated capacity of tests available," Screen 4 said.
Following Which's enquiries the headline listed prices of several firms rose.
However Which? said the underlying problem had not been corrected, as subsequently other firms were still quoting prices for a single test, making it appear that tests are available at half the true cost.
Which? also found that among the DHSC's accredited providers, several were not yet able to provide tests. Following Which?'s enquiries three firms were removed from the list.
The consumer group said taken together the problems suggested a lack of regulatory oversight and called for the government to explore options for reducing the cost of testing and to ensure providers were offering accurate information.
A DHSC spokesperson said: "The government regularly evaluates all providers' performance, including their delivery and test turnaround times.
"Providers who do not meet the minimum standards set by DHSC and reviewed by the independent United Kingdom Accreditation Service (UKAS), or where our monitoring highlights concerns about their customer service performance, are removed from the gov.uk travel test list. Additionally, providers may not appear on the list where they are temporarily out of stock."