Some international arrivals to the UK have waited nine days for Covid test kits that should be taken on day two of a 10-day quarantine period.
Travellers from countries not on the UK's "red list" must purchase a £210 testing kit, with a test taken on day two and day eight of isolation.
Those who do not take the tests may face penalties of up to £2,000.
A government spokesman said there were "no specific issues" with the deliveries or fulfilment of test kits.
They advised that customers should receive the test kits, providing the correct details are given at the point of booking.
However, several passengers have told the BBC that despite ordering the tests, they never arrived.
The social media pages for the testing provider, Corporate Travel Management (CTM), contain other comments complaining about delivery delays.
Yury Babin travelled to the UK from Moscow for work. He purchased the test kit on Friday on his arrival. Although he received an email confirming his order, he is still waiting for his tests four days later.
"When I called the line said: 'We are experiencing high volumes of calls', before dropping,' says Mr Babin. "I have emailed too, but had no response. I can't go to the pharmacy to buy the test, what are my options?"
Mr Babin describes the situation as "very frustrating".
"Of course I'm concerned that I might get a fine, but if I do I will challenge it. Why are the government making policies that they can't administer?"
He has now been told by the test provider, CTM, that his kit will be resent.
Lorenzo Calonghi lives in France, but returned to the UK in February to study engineering. He booked his tests on 20 February, but became worried when he hasn't received the first test by his second day in quarantine at home.
"On day three (of home quarantine), I called the company multiple times and was unable to reach them," says Mr Calonghi. 'I continued to call every day but still never received a call back - nor was my call ever picked up."
He eventually found out by contacting the company on Twitter that his order had been declined, although he had already received a confirmation email and been able to go through UK border controls.
He says that the company did not tell him that there was an issue with his payment and nine days on, he still has not received the tests.
Others have had similar problems. Tasadaq Hussain booked the testing package for his wife who was returning from Pakistan. When the tests did not arrive, they tried to reach the company.
"I was on hold for an hour and a half without an answer," he says. "We kept trying every day."
Mr Hussain eventually created a Twitter account and eventually received a response from the firm after eight days.
Anyone experiencing a delay in their test kit being delivered is advised to call 119.
BBC News has approached the testing provider CTM for comment.