The UK's latest testing and tracing figures have been labelled "surprising and worrying" after it was revealed the system has failed to reach thousands of people infected with coronavirus since it began.
While a majority of those who test positive are reached (more than 70%), NHS tracers have failed to get hold of a huge 24% of people since May 28.
It means at least 5,062 people with Covid-19 did not provide their close contacts to the NHS between May 28 and June 17.
Between June 11 and June 17, just 82% of close contacts were reached, down from 91% in the first two weeks.
"One in four positive people can’t be contacted – this is surprising and worrying – these people need to provide details to get the result of their test and they have a responsibility to be contactable," Keith Neal, Professor of Epidemiology of Infectious Diseases, University of Nottingham, said
“There has been a drop in the percentage of contacts who have been contacted from 90% to 80%, but this is still not a bad figure."
“Every contact contacted is a possible chain of transmission stopped," he added.
The prime minister’s official spokesman said the service was continuing to make improvements, but did not rule out “further measures” if people failed to co-operate.
“If we do find that significant numbers aren’t following isolation instructions we will consider whether further measures are required.”
With thousands slipping through the cracks, critics would say the figures contradict Boris Johnson's claim in response to Sir Keir, that the UK has an "effective cluster-busting operation" designed to keep outbreaks under control.
But the prime minister was right to claim NHS Test and Trace is reaching the "vast majority of those who test positive and their own contacts".
Since the service was set up 113,925 people were identified as recent close contacts and reached through the contract tracing system.
This was 89% out of a total of 128,566 identified contacts.
Despite Mr Johnson calling it a "formidable achievement", it appears the system's success rate has fallen since the last time figures were released.
In the first two weeks 73% of those with positive tests were reached and asked to provide details of recent contacts, but the most recent figures from June 11 to June 17 show just 70% of people were reached.
Reporters on a phone call briefing with Dido Harding, the person leading the UK's Test and Trace programme, questioned whether the absence of an app was the reason so many people could not be contacted.
The much-awaited NHS coronavirus testing and tracing app was heavily delayed before officials abandoned their plan to produce it in-house.
NHSX - the tech wing of the health service - is now working in conjunction with Apple and Google to produce the app after the original version, piloted on the Isle of Wight, encountered technical issues.
Baroness Harding said "no app is better than an app that doesn't work".
She said the "drop out rate" - people who could not be reached - was more down to issues such as language barriers and working schedules meaning people could not answer the phone.
"There's a lot we are doing to improve it," she said, "We are calling up to 10 times a day for one individual.
"We have increased our translation services, we have included multiple languages including sign language."
In a statement she said: "The strength of NHS Test and Trace lies not just in our thousands of trained tracers, but in the public playing their part – providing us with the vital information we need to stop the spread of coronavirus, protect our families and communities, and ultimately save lives."
She added: "We also need everyone who is contacted by NHS Test and Trace to respond to our calls, texts or emails and to follow the advice the receive.
“As we have seen from the start of this pandemic, it will take all of us working together to stop the spread of the virus in our communities and that will continue to be true as the country starts to open up again.”
The number of people not reached includes those who the service was unable to reach because there had been no response to text, email and call reminders.
It also includes people who were reached but declined to give details of close contacts.
Since May 28 3% of people (681) could not be reached because their communication details had not been provided.