At least a quarter of people who test positive for COVID-19 in the UK are being missed by contact tracers following up who they could have transmitted the virus on to.
Of the 5,949 people who tested positive for coronavirus between 4 and 10 June, 73% provided details of who they had been in close contact with, according to the latest NHS figures.
But of the 44,895 people who were identified as close contacts of these people - and potentially at risk of having caught COVID-19 - more than 40,900 (90%) were reached and asked to self-isolate.Government defends track and trace amid concerns
The figures mean that at least a quarter of people who test positive for the virus are being missed because there is a disparity between the numbers of people testing positive and the numbers being referred to the contact tracers.
Yesterday, Health Secretary Matt Hancock explained the disparity was "largely because they are in-patients in hospital and therefore testing and tracing in the normal sense doesn't apply".
But the previous week Professor John Newton said there was "quite a lot of double counting in the number of cases reported", and Baroness Dido Harding has confirmed there are "errors in the data".
According to the data, the proportion of people who are missed by the contact tracing teams has remained largely unchanged from the week before.
In total, since 28 May when the track and trace system was rolled out, more than 14,000 people who have tested positive for COVID-19 have had their cases transferred to the tracing teams.
Of these, 72% have been reached and asked to provide the details of their recent close contacts, 90% of whom the tracers were able to contact and ask to self-isolate.
Matt Hancock had promised that a contact tracing app developed by the health service's innovation arm NHSX would be delivered by mid-May, allowing this manual process to be automated.
But development of the NHS app, which was being trialled on the Isle of Wight, has been fraught with delays and concerns about whether it works properly.
Yesterday, Lord Bethell told a committee of MPs it may not be ready until the winter.
But, he told a committee of MPs, the app wasn't "the priority at the moment".
Lord Bethell confirmed the government still planned to introduce a contact-tracing app, describing it as "a really important option for the future".
The app has been the subject of a trial on the Isle of Wight, where the Department of Health says it has been downloaded by 54,000 people.
Lord Bethell said the trial had been a success, but admitted that one of its principal lessons had been that greater emphasis needed to be placed on manual contact tracing.
"It was a reminder that you can't take a totally technical answer to the problem," he said.