Test to Release, a new system meant to cut quarantine times for travellers arriving in England, has been beset with problems on its first day.
Travellers are allowed to end self-isolation early if they pay for a coronavirus test and get a negative result five days after arriving.
The government picked 11 firms to carry out the private tests.
However, some of the largest Covid test providers were not included and many on the list have hit problems.
The BBC understands that the Department for Transport did not accredit the larger operators who provide Covid tests at scale, because they ran out of time.
Instead, smaller operators were given accreditation.
When the scheme launched on Tuesday morning, some test providers were overwhelmed with the sheer volume of demand from the public. One supplier has pulled out completely.
One company, SameDayDoctor, asked to be withdrawn from the programme after being inundated with requests for tests.
It posted a message on its website stating: “Unfortunately we have been so overwhelmed with requests for Test to Release that we cannot answer any more emails nor take any more bookings.”
Another, Axiom, said it couldn’t take bookings but an update would be available “soon”, while another, Medicspot, told visitors to its website to register their interest.
The Prime Minister’s official spokesman told the Press Association: “We have made this option available to international travellers and we are working to approve more test providers.”