Downing Street has said there was a short delay in England's test-and-trace system asking some people who tested positive to provide details of individuals they had been in contact with.
It comes after a BBC report claimed failures in test-and-trace were partly responsible for a surge in the Indian variant in one of the worst affected parts of the country.
A report found that eight local authorities in England did not have access to the full data on positive tests in their area for three weeks in April and May, the BBC said, and the highest number of missing cases was in Blackburn with Darwen, Lancashire.
Blackburn is one of the worst hit places in the country for the Indian variant with health secretary Matt Hancock warning it is "becoming the dominant strain".
Other areas affected by what is thought to have been a technical glitch were Blackpool, which like Blackburn is in the North West, York, Bath and North East Somerset, Bristol, North Somerset, Southend-on-Sea and Thurrock.
The prime minister's official spokesman told reporters on Thursday: "In this specific instance, all positive cases were contacted and told to self-isolate for 10 days.
"As you know, there was a short delay when asking some of those positive cases to provide details of individuals they had contacted since contracting COVID.
"This issue was across a small number of local authority areas and was quickly resolved."
Asked whether the failure had contributed to the spread of the variant, the spokesman said: "The spread of the variant will be down to a number of factors - this was an issue that occurred across a small number of local authority areas, so I don't think it is possible to draw that conclusion from this."
The BBC reported that between 21 April and 11 May, only a limited number of positive cases of coronavirus were reported to the eight local authorities.
Then, on 11 May, the Department of Health and Social Care (DHSC) told them that, over that period, 734 positive tests had not been reported.
Labour and the Liberal Democrats said the government has questions to answer.
The Lib Dems' Layla Moran, chair of the all-party parliamentary group on coronavirus, said: "The government has serious questions to answer over how failures in its Test and Trace system have contributed to the surge in cases of the Indian variant.
"It appears that not only did ministers fail to prevent the Indian variant entering the UK, but that grave errors were made in tracing cases in the hardest-hit areas.
"Matt Hancock talks about the government having a sword and shield against the coronavirus, but their approach has more in common with a leaky colander.
"The government must urgently confirm the number of cases that were missed and what is being done to prevent this from happening again. They need to also finally fix the failing Test, Trace and Isolate system, including by providing proper financial support to those self-isolating."
There has been a "rapid rise" in cases of the Indian variant in the North West, the area's regional director at Public Health England has said.
Andrew Furber told Sky News: "We are seeing a rapid rise in cases in Bolton and in Blackburn, and we are now seeing a spread further afield to Burnley.
"So we are beginning to see it spread much further throughout the North West."
Testing and vaccinations have been "turbo-boosted" in these areas, he said, adding that authorities wanted to see people "literally in their thousands" turn up to get the jab.
Because of its sharp rise in COVID-19 infections, the vaccination programme has been bolstered in Blackburn and Darwen.
An extra 1,000 Pfizer doses were secured every day for the area for the two weeks from last weekend.
On Sunday, Mr Hancock didn't rule out imposing local lockdown restrictions in places worst affected by the Indian variant of coronavirus, including Blackburn and Bolton.