The PM says England will have a “world-beating” tracing system from June, as he was accused of leaving a “huge hole” in the country’s coronavirus defences.
Boris Johnson said 25,000 contact tracers, able to track 10,000 new cases a day, would be in place by 1 June.
BBC health correspondent Nick Triggle said it was unlikely to be a “fully-functioning perfect system” by then.
Labour leader Sir Keir Starmer challenged the PM over the absence of a tracing system since March.
Contact tracing is a system used to slow the spread of infectious diseases like coronavirus, and is already being used in Hong Kong, Singapore and Germany.
One method involves the infected person listing all the people with whom they have had prolonged and recent contact, to be tracked down by phone or email.
Another uses a location-tracking mobile app, which identifies people the patient has been in contact with.