More than 1,300 people were wrongly told they had coronavirus due to a lab error with NHS Test and Trace.
The Department of Health and Social Care (DHSC) said 1,311 people who took a test between November 19-23 across the UK were incorrectly told they received a positive result.
It said there was an issue with a batch of testing chemicals which meant their results were void.
A DHSC spokesman said: "Swift action is being taken to notify those affected and they are being asked to take another test, and to continue to self-isolate if they have symptoms.
"This laboratory error was an isolated incident and is being fully investigated to ensure this does not happen again."
The DHSC did not comment on whether the error affected regional figures of infection rates.
Recently it emerged the widely criticised system is still failing to reach two in five coronavirus contacts and remains near a record low.
Of the 347,575 close contacts of someone who has tested positive in the week to November 18, only 60.3% were reached and told to self-isolate.
This is down slightly from 60.7% in the previous week, and is also just above the all-time low of 60.1% for the week to October 14. It is well below the 80% target.
Positive Covid-19 cases in England fell 9% in the latest week, the first week-on-week drop since the summer.
For cases managed by local health protection teams, 99.0% of contacts were reached and asked to self-isolate in the week to November 18.
Earlier this week figures revealed about four in 10 contacts of those who test positive for the virus are still not being reached.
The Prime Minister announced an extra £7 billion for Test and Trace in his winter plan on Monday in a bid to increase testing and improve contact tracing.
It takes the overall funding provided for Test and Trace this financial year to £22 billion - nearly a fifth of the entire annual NHS budget - which has been met with some criticism.
But Boris Johnson was adamant the system provided "value" for money although admitted there had been "teething problems".
