The spread of the Indian coronavirus variant was made worse by a failure in the test and trace system which meant eight councils didn't know about more than 700 positive test results, according to a report.
The BBC reported the eight English councils did not have access to full coronavirus infection data between 21 April and 11 May.
This meant 734 positive tests were not registered, according to the report, meaning their contacts were not traced locally and told to self-isolate.
Among those councils was reportedly Blackburn with Darwen, which has among the highest number of cases of the B.1.617.2 variant in England.
According to the report, the other council areas affected were Blackpool, York, Bath and North East Somerset, Bristol, North Somerset, Southend-on-Sea and Thurrock.
Officials at one of the councils reportedly said the spread of the variant was “exacerbated by the sporadic failure of the national test and trace system”.
Cases involving the variant have accelerated across England in the past week, with 2,967 reported on Wednesday. This was up 1,654 from 1,313 on Thursday last week (13 May).
Watch: Almost 3,000 cases of Indian variant in UK
Downing Street labelled the issue a “short delay” on Thursday and said problems with informing eight councils about the positive cases were “quickly resolved”.
Boris Johnson’s official spokesman told reporters: “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 government accepted the failure 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.”
Labour shadow health secretary Jonathan Ashworth said: "It beggars belief that yet again local health experts on ground have been left in the dark for two weeks [sic] when we know acting with speed is vital to containing an outbreak.
“Ministers need to explain what’s gone wrong and provide local health directors with all the resources they need to push infections down.”
Since last week, the spread of the variant has led to questions about whether the government will be able to go ahead with its plan to drop all restrictions on social contact in England on 21 June – though Boris Johnson has signalled COVID-19 vaccines are effective against the strain.
Watch: Government has 'increasing confidence' vaccines effective against Indian variant