More than 7,200 people in England were told to stop self-isolating on the wrong date by the Test and Trace scheme as a result of a software error.
The Department of Health said most of those affected had subsequently been contacted with the correct information.
People had been told to isolate for too long, rather than being told they could mix with others too soon, it added.
The mistake – which was first reported by Sky News – follows a series of other software-based Covid-19 foul-ups.
Last month, the BBC revealed how an oversight in the use of Microsoft's Excel software led to nearly 16,000 coronavirus cases going unreported in England.
And on the weekend, the Sunday Times reported that a risk-score threshold used by the NHS Covid-19 app to trigger self-isolate alerts had been lowered weeks later than intended. In that case, officials are still carrying out checks to identify the "root cause".
A total of 7,230 individuals were involved in the latest error.
Officials believe it resulted from an internal update to the system used by human contact tracers, who identify people believed to have recently been close to those diagnosed with the coronavirus.
The system is used to calculate how long the original person who tested positive should keep away from others. It also does the isolation calculation for those they had been in close proximity to, who are contacted via follow-up phone calls, emails and/or text messages.
It is completely separate to the automated contact tracing system used by the app.
The update was made on 22 October and affected a total of 7,230 people before the problem was rectified on 27 October.
4,775 people still in self-isolation were later contacted with the correct end date2,455 individuals had already ended their self-isolation
"We have reassessed the self-isolation periods for a number of people who were contact traced, following close contact with someone who tested positive for Covid-19," a Department of Health and Social Care spokeswoman said.