Tens of thousands of Britons had to be retested for coronavirus after the government sent their swabs to the US.
Nearly 30,000 tests were found to be invalid after being flown out following problems at a UK laboratory last month.
The 29,500 voided tests were among a batch of 67,000 sent to the US following technical issues at the privately run Randox laboratory in Northern Ireland.
It emerged last month that the government had to send 50,000 tests abroad owing to processing problems in the UK. However, it has now emerged that 17,000 more tests than previously admitted were sent to the US and that 29,500 came back void.
Samples have to be tested within 72 hours of the test being taken, which means that any delay in their processing could leave people with symptoms unsure if they have the virus.
On 18 May, Matt Hancock announced coronavirus testing is being extended in the UK to anyone over the age of five with symptoms.
Before then, eligibility had been limited to a series of groups including key workers, those aged over 65, people who could not work from home, or people who lived with someone from one of these groups.
Those with symptoms can now use the gov.uk website to book home test kits or appointments at drive-through centres across the country.
The Telegraph reported that the 67,000 tests were airlifted to a university lab on the east coast of the US over 10 days last month after officials failed to find capacity for them to be analysed in the UK.
The swabs were returned a few days later in two large bags, the paper reported, one containing a “much higher than expected void rate”. It said part of the problem appeared to be different equipment standards in the US.
The Department of Health and Social Care said: “We worked hard to get complete tests for people under difficult circumstances. In many cases that worked and we are grateful for the team for their efforts.
“But in some cases it didn’t, and the correct judgment was made to void the tests. Everyone affected was offered a new test immediately and we worked quickly to restore full capacity in the UK.”
Lisa McNally, the public health director at Sandwell council in the West Midlands, said on Thursday she was surprised the lockdown had been eased before the test-and-trace system was “at full speed”.
She told BBC Radio 4’s Today programme: “What surprises me, I guess, is that we are moving ahead so quickly with easing lockdown before the system is at full speed. We need to allow time to assess how things are going, get this system up to speed.
“To be honest with you, I’m concerned that by the time we realise we need to hit the brakes, we will have spun off the road and be over the cliff-edge.”
McNally predicted the UK could be “going back to a place we really don’t want to be, which is where we were at the end of April” in terms of the number of coronavirus-related infections and deaths if test and trace did not prove effective.
On Wednesday Boris Johnson committed to getting “all tests turned around in 24 hours by the end of June, except for difficulties with postal tests or insuperable problems like that”.
He made the promise in the Commons after the former health secretary Jeremy Hunt said getting tests done within 24 hours was “absolutely essential” to the success of the government’s much-vaunted test-and-trace system.
The government’s Scientific Advisory Group for Emergencies (Sage) has said the system must get infected people’s contacts to isolate within 48 hours to be effective. However, tests currently take 48 hours on average to produce results.