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 are 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 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”.
The revelation comes 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, 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.
When asked if the rate of new infections had to be down to hundreds a day for the strategy to be effective – when it is currently in the thousands – Prof Van-Tam said it was entirely appropriate for it to be part of the “overall measures” to tackle the virus.
Stressing it was not the total solution, he said it would give more room for steps to ease social distancing.