The co-founder of the Covid testing centre at Gatwick Airport has apologised for "frustrating" delays in its service.
Nick Markham, from ExpressTest, said it had been "overwhelmed by demand".
Passengers have complained of having to delay travel plans because they could not get their results back in time.
ExpressTest has said it is hoping to increase the capacity of its test centre at Gatwick's South Terminal car park in the next week.
Roger Cooper, from Bexley, had to delay his flight to Fuerteventura by a week because of difficulties booking his test.
He said: "They just kept promising they were going to call back to make the appointment.
"I was told they had plenty of available slots even though the website was not working."
Malcolm Weston, from Hampshire, was also travelling to Fuerteventura, and described the testing process as "a nightmare".
"The stress was unbelievable", he said. "I got a test on Thursday 3 December, to travel on Saturday 5.
"I finally got the result at 01:30 GMT on Saturday. I had to leave for the airport an hour later. I almost cancelled the trip."
Mr Markham said about 120,000 people had tried to book a test within the first few days of them opening on 27 November.
He added: "I apologise to those people who have had trouble getting bookings. I know it has been frustrating for people."
He said that they have since tripled capacity in their call centres.
The screening facility is available for people who require a valid, negative Covid-19 test certificate up to 96 hours before travel.
From 15 December passengers arriving in England from countries not on the government's travel corridor list will have the option to take a test after five days of self-isolation, with a negative result meaning they will no longer need to be in quarantine.
Passengers and airport staff are charged £60 for a test, with the general public being asked to pay £99.