The government has admitted sending about 50,000 coronavirus tests to the US last week for processing after “operational issues” in UK labs.
The Department of Health said sending swabs abroad is among the contingencies to deal with “teething problems”.
The samples were airlifted to the US in chartered flights from Stansted Airport, the Sunday Telegraph said.
Results will be validated in the UK and sent to patients as soon as possible.
A spokeswoman for the Department of Health and Social Care (DHSC) said expanding Britain’s virus testing network had involved setting up an “entirely new” lab network to process tests, adding “contingencies” – such as sending swabs abroad – were in place for when “problems arise”.
Communities Secretary Robert Jenrick said the kits had to be sent abroad because of a “temporary failure” at a lab.
He told the BBC’s Andrew Marr Show that the move “shows our determination to get the job done”.
Meanwhile, the government has sent an urgent alert to hospitals recalling 15.8m protective goggles due to safety concerns.
Although the “Tiger Eye” protectors, purchased in 2009 during the swine flu pandemic, were in CE marked boxes – meaning they should have met European Union safety requirements – the goggles have since been retested and do not provide proper splash protection.
Commenting on the recall, which was first reported in the Sunday Telegraph, a DHSC spokeswoman said the safety of front-line staff was “our top priority”.
She added that hospital trusts should have enough goggles to “immediately stop” using the “Tiger Eye” protectors. A further 9.2m of the goggles are in quarantine, she added.
The revelations come as the government failed to hit the 100,000 daily tests target set by Health Secretary Matt Hancock for the seventh day in a row.
There were 96,878 tests delivered in the 24 hours up to 09:00 BST on Friday, down from 97,029 the day before.
But health leaders said they expected “fluctuations” in the figures, and that testing was still much higher than it was at the start of the outbreak.
Speaking at the Downing Street coronavirus briefing on Saturday, deputy chief medical officer Prof Jonathan Van-Tam said he expected “fluctuation” in the day-to-day figures.
He said: “We are now really at a high plateau, in the region of 100,000 tests per day.
“I don’t think we can read too much into day-to-day variations, but the macro picture is this is now at a much, much higher level than it ever was at the beginning of this crisis.”
BBC health reporter Rachel Schraer said the UK did not start with the resources to do mass testing, unlike some other countries.
But it also took several weeks to expand from an initial eight public health laboratories to a wider network of private and university labs.
Unlike the UK, countries like Germany and South Korea rapidly stockpiled kits and made the test available to a larger number of labs.
Prof Van-Tam also told the briefing that the test-and-trace strategy of finding people with the virus and tracking people they have been in contact with was “part of the solution” needed to ease the lockdown.