Private companies have been accused of cashing in on the coronavirus pandemic by offering expensive Covid tests to customers with urgent travel plans, then failing to deliver.

Most countries now require travellers to be tested up to 72 hours before they fly and dozens of unregulated companies have sprung up offering DIY swabs or on-site tests with results promised within 48 hours.

However, airline passengers who paid up to £330 for a Covid-19 travel certificate have reported missing flights because their test results did not arrive before their departure.

Related: Negative Covid test required to enter England from Friday

Violeta, who did not want her full name published, never received her test kit after paying £82.99 to a startup firm, Confirm Testing. She was unable to make contact with the company and her flight departed without her. “I lost the cost of my flight tickets and, a month on, I’ve not had the test fee refunded,” she said.

The firm, which was registered last April, offers next-day self-test kits with a pre-paid return label. Results are promised within 48 hours of their return.

Another reader, who did not want to be named, paid £149 to test provider Medicspot three days before she was due to fly to Spain for surgery. It promised results within 21 hours but she had still not received them by her departure date and was unable to fly.

“I lost my hospital appointment and the cost of the four scans and pre-op drugs was therefore wasted,” she said. “Getting through to Medicspot customer service was nigh impossible.”

She has been refunded for the test, but has not been able to get back more than £1,000 in medical expenses.

Last month, the government removed the requirement for commercial Covid testing providers and partner laboratories to be regulated by the Care Quality Commission.

Instead they have to apply for accreditation with the United Kingdom Accreditation Service (UKAS) and self-declare that they comply with government guidance. However, applicants have until late June to complete the accreditation process, which means that many companies are selling services without official oversight.

The Department for Health and Social Care publishes a list of firms who have self-declared that they meet minimum standards, but states that it does not endorse any of them.

Nonetheless, a number claim to be “government-approved”.

Another reader, Jennifer, paid a £249 fee for a pre-flight appointment at Vivo Clinic, a cosmetic treatment specialist which now offers on-site private Covid testing.

“The results were promised within 48 hours but I only realised when I got there that that was 48 working hours,” she said. “This was too late for my flight so I had to pay another £84 for next-day results.”

She was required to administer the test herself and did not receive the results until days after she was due to fly. The company blamed a mistyped email address and refused to refund the £333.

Vivo Clinic informs complainants on TrustPilot that Care Quality Commission rules forbid its staff to administer the tests during appointments. The Care Quality Commission, which no longer regulates the sector, told Guardian Money there has never been any such ban. Vivo Clinic did not respond to a request for a comment.

Confirm Testing did not respond to a request for a comment, but on its website it said it had suspended its PCR testing service, blaming Royal Mail delivery delays.

Medicspot, which says its Covid testing service is government-approved, blamed a “severe backlog” at its partner lab for its service failures.

“We stopped taking new orders for Covid-19 tests as soon as we became aware of the problem and we have offered all customers affected a full refund,” it said.

“We will not resume the service until we are confident we can deliver results on time and provide improved access to our dedicated customer service team.”

The consumer group Which? warned that the problem was likely to get worse when the travel market reopens with some fake websites selling appointments and swab kits that don’t exist.

The Which? travel editor, Rory Boland, said: “With testing increasingly important for flights abroad, it’s vital that this sector is properly regulated to ensure passengers are not stung by rogue operators and shoddy service.”

A Department of Health and Social Care spokesperson said: “We have a robust process in place to ensure that all tests meet the required high standards.

“There is a complete end-to-end accreditation scheme, run by Ukas, which covers private sector providers of Covid-19 testing and this includes test providers of test to release, used for international travel.”