A government-listed Covid testing firm has apologised after issuing certificates showing a negative result to customers who did not take a test.
001Doctor.UK said its laboratory partner had issued an "erroneous batch" of test results.
It is the latest in a series of problems travellers have experienced with private Covid testing firms.
"We fully understand our customers' frustration and we are sorry they were let down," said a 001Doctor spokesman.
"The links to any erroneous results were invalidated rapidly, and the supplier responsible for this error has contacted all those [this had an impact on] to apologise."
Anyone coming to the UK from abroad has to pre-book Covid tests, to be taken on day two and day eight after their arrival.
But a surge in demand in late March led to several of the firms providing the tests scrambling to fulfil the orders they had taken.
Customers were left feeling cheated when tests which cost up to £200 were late or failed to arrive.
001Doctor, in particular, was the target of a tsunami of complaints online.
Shortly afterwards, the company was removed from the Department of Health's list of suggested providers of day two and day eight tests. But it remains on a government list of providers for general covid testing.
But in recent days, a new problem came to light when customers like Christiana Raymond received Covid results via email.
Ms Raymond, a key worker who had been to France to attend a funeral, ordered tests through 001Doctor.
After a delay, she received the first test which she returned. It came back negative.
The second test kit was also delayed but, after getting tested by the NHS, Ms Raymond decided not to use the 001Doctor test.
So she was shocked when she received a certificate from the company telling her the result of her second test was also negative.
"I'm speechless," Ms Raymond said. "I can't quite believe that they would have sent an email to someone who has not sent the test kit back. I mean, that's just basic isn't it?"
She said she was "very alarmed about what's going on with this system and that there doesn't seem to be any accountability for this" especially as overseas travel from England is set to open up after 17 May, which will boost demand for tests further.
Earlier this month 001Doctor said the problems it had encountered had been "beyond its control", including the high volume of orders, and problems with delivery services such as Royal Mail.
It declined to say how many certificates had been issued as a result of the erroneous batch of results, sent from its laboratory partner, BioGrad Diagnostics.
BioGrad Diagnostics, a Liverpool-based firm which provided 001Doctor with Covid kits as well as processing the tests, said: "A small number of erroneous results were issued due to an administration error."
But a BioGrad spokeswoman added: "While we have apologised for our part in this issue, the certification issued by 001Doctor did not have unique run ID, which is only added when the test has been run in the lab, so this should have been picked up in [001Doctor's] own quality processes."
It was the first time the two businesses have worked together, though BioGrad said it was reviewing its relationship with 001Doctor "in light of these issues".
A spokesman for 001Doctor said: "The run ID is an internal process element related to the supplier and was not mentioned to 001Doctor until after this issue arose. It has no impact on our own quality process."
Business executive Daniel Everett is another customer of 001Doctor who ordered the day two and eight tests before the company was removed from that list.
Prior to travelling from Switzerland to the UK, he paid more than £180 for the two Covid kits.
Neither arrived for the entire two weeks he was in the country.
And yet, during that time he, too, received a certificate from 001Doctor, showing a negative result.
"They issued a document that could be legally used to demonstrate my negative status," he told the BBC.
Mr Everett was able to get his money back for the tests that were never sent, but only through his credit card provider in Switzerland.
Craig Dodd hasn't been so lucky. He says he is still owed £183 by 001Doctor and, despite never taking one of its tests, says he also received a certificate showing a negative Covid result.
"The whole system is a complete shambles," said Mr Dodd, adding that he could not understand why the government was allowing a company like 001Doctor - which was set up last November according to Companies House - to carry out this work.
"It was registered at Companies House only a few months back so why is the government giving them this responsibility?"
A spokesman for 001Doctor said: "The issues in question are entirely to do with exceptional demand for the day 2 and 8 day testing regime felt right across the industry.
"001Doctor has been operating its test-to-release and fit-to-fly services without issue".
Like Mr Dodd, Ms Raymond is concerned that private companies have been given responsibility for providing Covid tests for travellers.
"Words fail me," she said.
"There are lots and lots and lots of people who have to travel for valid reasons at the moment, who are reliant on this to travel and to get back into the UK safely, but then are also reliant on these tests to be accurate, to be able to go back to work and do their job."
The Department for Health and Social Care said: "We do not endorse or recommend any particular test provider for quarantine test packages and the government ensures all private providers of tests meet the minimum required standards.
"Tests and providers are both rigorously reviewed by the independent United Kingdom Accreditation Service (UKAS) and the list of private test suppliers is continually updated to ensure the providers listed meet the required high standards."
"The government carefully monitors issues raised by the public about private test providers and takes action where appropriate, including removing providers from the list."