Failures in England's Test and Trace system are partly responsible for a surge in the Indian variant in one of the worst affected parts of the country, a report seen by the BBC says.
For three weeks in April and May, eight local authorities in England did not have access to the full data on positive tests in their area.
The government said a Track and Trace "software issue" had affected a "handful" of places, but this had been resolved "as quickly as possible".
The other areas experiencing incomplete data were Blackpool, York, Bath and North East Somerset, Bristol, North Somerset, Southend-on-Sea and Thurrock.
NHS Test and Trace - for which £37bn has been set aside - identifies people who have been in close contact with someone who has caught Covid.
Between 21 April and 11 May, the system only provided details of a limited number of positive cases of coronavirus to the eight local authorities.