The resumption of international travel could "overwhelm" the UK’s testing system, consumer champion Which? has warned.
Which? said testing providers were already struggling to meet demand, even with huge restrictions on overseas travel.
Anyone travelling to the UK must currently self-isolate for 10 days and take PCR tests on day two and eight of their quarantine. Both tests must come back negative before they can leave self-isolation after the tenth day.
However, Which? said some passengers were having to extend their quarantine, or pay for additional Covid tests, with some private labs failing to deliver test results on time.
Which? said this raised concerns about the capacity of the UK’s travel testing system – just weeks before a limited resumption of international travel from 17 May.
Under the government’s new traffic light system for the resumption of international travel, arrivals from "green list" countries won’t have to quarantine but will have to take a PCR test by the end of the second day of their return.
Arrivals from "amber" and "red" list countries will continue to have to take tests on day two and eight of their return while self-isolating, with red list arrivals continuing to have to enter a government-appointed quarantine facility upon arrival in the UK.
"With mass travel set to resume next month, Which? is concerned the travel test system, which is already struggling to handle demand when international travel is restricted, is at risk of being overwhelmed, and potentially collapsing when restrictions are lifted," said the watchdog.