The UK government has admitted sending about 50,000 coronavirus tests to the US last week for processing after "operational issues" in British labs, it was reported on Sunday.
The Department of Health said sending swabs abroad were among the contingencies to deal with "teething problems", the BBC reported.
The samples were airlifted to the US in chartered flights from Stansted Airport, the Sunday Telegraph said in a report.
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".
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 9 a.m. on Friday, down from 97,029 the day before, said the BBC.
Prime Minister Boris Johnson has said his "ambition" was to hit 200,000 tests "by the end of this month - and then go even higher".
As of Sunday, the overall COVID-19 cases in the UK stood at 216,525, with 31,662 deaths, which is currently the highest number of fatalities in Europe.
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