But experts said potentially “tens of thousands” of travellers have used the “loophole” to avoid buying a new test. Lateral flow tests cost around £30 each, but some PCRs can cost upwards of £100.

It is understood the form only checks that the code matches the correct combination of letters and numbers to be accepted.

The Telegraph has been made aware of examples where passengers have used the same test code on their PLF multiple times.

In one case, a traveller used the same code on two forms when travelling between three European countries.

Others have posted about the loophole on social media, with one traveller, based in London, tweeting: “It’s such a con and as you are able to reuse your Day 2 booking code time and time again on multiple PLF … it makes a complete mockery of the concept.”

The Laboratory and Testing Industry Organisation (LTIO) trade body announced on Friday that it had completed several trials which revealed that booking reference numbers for travel Covid tests can be repeatedly used and are not checked.

A spokesperson for the LTIO said: “This research completed by our organisation is simply shocking. It appears that there are no checks done on passenger locator forms by the Government, leaving the entire system vulnerable to abuse.

“A reliable testing system at our borders is vital in the fight against the omicron variant. But even the most basic check of ensuring travellers have a genuine PCR test booked is not being completed.”

‘Tens of thousands’ of passengers suspected to have used ‘loophole’

Paul Charles, the chief executive of travel consultancy The PC Agency, said: “It is a known issue. Obviously it’s not something that should be endorsed, but definitely some travellers have been doing it. I suspect tens of thousands of travellers have been doing it actually.”

Scientists have called for the loophole to be tackled, warning that it weakens the UK’s ability to limit the number of omicron cases entering the country.

Professor Paul Hunter, Professor of Health Protection at University of East Anglia, said: “Anything that allows people to game the system reduces the effectiveness.

“Clearly if people are not [taking the tests] when they should, that would increase the risk but how effective border controls are in controlling the spread of infection is uncertain.”

It is a criminal offence to provide false information on the PLF and passengers could face a fine if it is discovered the provided false details.

The Government said the test codes inputted into the PLF are monitored, but did not confirm to The Telegraph how or how many times the same codes have been used by travellers.

A government spokesperson said: “Our utmost priority is protecting the health of the public. Passenger Locator Forms have played an important role in enhancing our borders regime to help reduce the risk of new variants being transmitted.”