The head of NHS Test and Trace told MPs research had shown up to 40 per cent of people contacted by the programme were not fully isolating when told to do so.
Appearing before the House of Commons science and technology committee on Wednesday, Baroness Harding used last week’s total number of cases and contacts, around 700,000, and concluded that 20,000 a day are not isolating.
Responding to questions from former health secretary Jeremy Hunt, she said: “Could I add a slight complexity to your calculation, which actually might well make your number go up a bit, which is that’s the proportion that we know about.”
She explained: “Every person who self-isolates when they’re told to by NHS Test and Trace plays their part in breaking the chains of transmission.
“Clearly, for every individual that fully complies, the impact on R improves.
“The way that we combat the disease is through a number of different interventions that requires societal change and you know we’re all human beings, no-one’s perfect.
“And getting that consistent societal behavioural change is probably a life’s work rather than something that you can do in nine months.”
Mr Hunt responded: “Thousands of people a day is enough to restart the pandemic.”
Ms Harding told the committee there could be a string of reasons, including not understanding what they should and shouldn’t do, the practicalities of self-isolating, not being able to afford it when they need to work, or mental health concerns.
Baroness Harding also came under fierce criticism after telling MPs the virus mutating “was something that none of us were able to predict”.
Jonathan Ashworth, Labour’s shadow health secretary, demanded the UK Government act now to help people stay home.
He said: “Confirmation that 20,000 people a day sick with Covid are not isolating confirms our repeated warnings that without decent sick pay and support we won’t break chains of transmission.
“With worrying identification of the South African variant in the community and the E484K mutation in the infectious Kent variant, it’s now more urgent than ever that this hole in our defences is fixed.”