The launch of a new system to reduce quarantine requirements has been marred by problems with coronavirus testing companies.
From Tuesday, travellers arriving in England can end their self-isolation if they pay to take a coronavirus test at least five days after they land, and get a negative result.
But several of the 11 testing firms on a Government-approved list published on Monday night are facing difficulties providing the Test to Release service.
One company, SameDayDoctor, asked to be withdrawn from the programme.
It posted a message on its website stating: "Unfortunately we have been so overwhelmed with requests for Test and Release that we cannot answer any more emails nor take any more bookings."
Other firms were also not ready to deliver the service.
Axiom Laboratory told potential customers that "Test to Release in not yet available", Halo Verify said its tests were out of stock, while Medicspot was not accepting new orders due to a huge backlog.
People arriving in the UK from destinations on the Government's travel corridors list are exempt from the 10-day self isolation requirement.
The Test to Release programme – announced by the Department for Transport three weeks ago – was designed to benefit people arriving from other locations.
Paul Charles, chief executive of travel consultancy The PC Agency, told the PA news agency: "It's a chaotic start for a system that was flagged as a solution to recovery in the travel sector, but it's been weeks in planning and has taken minutes to fall apart."
There is "no excuse for the teething issues", he claimed.
"It's not the most difficult process to put together and remarkably it seems to have been cobbled together at the last minute," he said.
"I think most people just won't pay for a test because they can't guarantee they're going to get the results quickly, so they may as well just opt to spend two or three more days in quarantine and save the money.
"That's the opposite of what the Government envisaged."
Richard Burge, chief executive of the London Chamber of Commerce, said: "It defies belief that the Government's long-awaited aviation Test To Release scheme has, within hours, proved to be unworkable.
"The companies already used by London's airports are not on the list of approved providers. But we now hear reports the Government's approved testers are unable to provide tests, or have no websites, and or are unable to take bookings for a test.
"The return of international business travel and tourism is critical to London and the UK's economic recovery. This requires competent and proven testing companies.
"The Government must urgently remove its selected providers who have fallen at the first fence and add those that can and already are providing testing services at our airports.
"This is another example of Government doing things to business rather than with business. We need fewer edicts from Whitehall and more genuine collaboration with the front line."
The Prime Minister's official spokesman said: "We have already secured 11 testing providers that meet our minimum standards and are working at pace to secure more."
Asked whether Boris Johnson was happy with the way the scheme had been launched, the spokesman said: "We have made this option available to international travellers and we are working to approve more test providers."