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.
Which? highlights a number of cases in which individuals haven’t received their testing kits or their results, flagging growing numbers of complaints on social media and review sites – including a Facebook group for those experiencing problems with 1,500 members.
It said despite some of the troubles, many affected passengers were not receiving refunds.
Which? noted how, at present, only four of the more than 500 private test providers on the government’s list of providers have been accredited, with only two – Oncologia and Nationwide Pathology – offering day two and day eight test kits.
One provider apologised for delays following an "unprecedented increase in Covid testing enquiries and kit orders since government travel rules were introduced".
Both Oncologia and Nationwide told Which? they didn’t guarantee test results within 10 days.
Which? said the government must "urgently address the problems" affecting the PCR testing market, including availability of information, accreditation, pricing and accessibility of tests, as well as levels of service from some providers.
It is also urging the government to ensure there are effective consumer protections in place for travellers in the event of any problems with testing ahead the resumption of travel from 17 May.
Rory Boland, Which? Travel editor, said: "The UK’s travel testing system can’t cope with demand, even when relatively small numbers of people are travelling. It’s clear the system could buckle under the pressure when mass international travel restarts and hundreds of thousands more people are reliant on it.
"Travellers shouldn’t have to shop around for something as crucial as a test provider – they simply need a service that is accessible, reliable, and delivered on time. It is critical the government addresses issues with testing ahead of restarting international travel, and ensures travellers are not left to the mercy of poor quality providers or unreliable services when trying to do the right thing in following government requirements to travel safely."
Both Oncologica and Nationwide cited postal delays for issues experienced by customers; Royal Mail said there were no reported delays with its network related to people using priority post boxes for test kits.
"For overseas travellers, the management of test kits, samples and results is carried out through several private companies," said Royal Mail. "In some instances, some of these companies may use a Royal Mail tracked returns service to deliver test results. In such cases, we deliver mail items in line with the service specification agreed with the sending companies."