England's Test & Trace system had its busiest ever week at the end of October but reached a smaller proportion of at-risk people than at any point since it started.
The contact-tracing programme, which phones, texts and emails people who have been close to someone who later tested positive for Covid-19, failed to get hold of 40 per cent of those at-risk people in the week from October 22 to 28.
The 59.9 per cent of contacts who were successfully reached and told to self-isolate was the lowest percentage since the system started in May.
It dropped from 61 per cent the week before and 60 per cent the week before that, and has plummeted from a high of 91 per cent when the programme began.
As England's second wave rumbles on and the country today enters its second nationwide lockdown, the system is crumbling under the sheer numbers of people who are testing positive and being referred to contact tracers.
The system, headed up by call centres operated by private contractor Serco, had to try and get hold of 327,203 contacts in the most recent week when 139,781 people were referred after testing positive. This was more than in any other week.
Its lacklustre performance means that 131,136 people who might have been carrying Covid-19 without knowing it were never told by officials.
The bungling system, which is privately-run but bears the name of the NHS, has hit repeated roadblocks since its inception, including software failures, staffing problems, laboratory backlogs and poor compliance from the public.
One statistics expert from Oxford University, Professor James Naismith, said he wasn't convinced Test & Trace was having a 'meaningful impact on the disease'.
In a glimmer of good news for the well-paid bosses at the helm of the system - some of whom earn £7,000 per day - the time it takes to get people their test results back improved in the most recent week across all parts of the programme. The total number of tests completed, however, was down on the previous week, despite positive cases being higher.
Although the Test & Trace system saw a record number of cases in the last week of October (137,180), the number of tests actually declined, to 395,771 over the course of the week, down from 421,615 the week before.
Because tests were down - the reasons for which aren't clear - the positivity rate has surged to 9.3 per cent, with 10.9 per cent in community testing.
This is cause for concern because it means a growing proportion of people who think they have Covid-19 actually do, and tests are struggling to find everyone.
Professor Naismith said: 'Until we have the ONS data tomorrow that cover last week, a definitive assessment on the efficiency of the system for last week is not possible.
'Based on what I see in this release I would not expect to change my previous assessment that system is not meaningfully impacting upon the disease.
'I note that the positivity rate in pillar 2 has gone above 10 per cent. Pillar 2 is testing the wider community.
'At above 5 per cent positivity, daily test numbers run an increasing risk of underestimating the growth of the virus. This is well understood by scientists...
'I have no idea why, given the positivity in pillar 2, we are operating pillar 2 so far below the Government’s measure of capacity.'
The Department of Health claimed success today when it pointed out that more 10million people have now been tested at least once for Covid-19.
Dido Harding, the chair of Test & Trace, said: 'Latest figures show NHS Test and Trace is processing more tests and reaching more people, which means we are finding positive cases and helping to break chains of transmission.
'We know that there are areas where we still need to improve and we are working tirelessly to make the service quicker and more effective every day.
'We have expanded capacity to over 500,000 tests a day to help meet demand over the winter period and continue to improve test turnaround times. Increased capacity and testing innovations have also allowed us to start regular testing of staff across the NHS and care homes, which will start tomorrow.
'NHS Test and Trace is a crucial weapon against this virus, but it is not a silver bullet. I urge everyone to follow new restrictions and book a test if you have symptoms to help protect each other during this time.'
For cases managed by local health protection teams, 97.9 per cent of contacts were reached and asked to self-isolate in the week to October 28, Department of Health data showed today.
For cases managed online or by call centres, the figure was 58.5 per cent.
Some 26.4 per cent of people who were tested in England in the week ending October 28 at a regional site, local site or mobile testing unit – a so-called 'in-person' test – received their result within 24 hours.
This was up from 22.6 per cent in the previous week.
Prime Minister Boris Johnson had pledged that, by the end of June, the results of all in-person tests would be back within 24 hours.
He told the House of Commons on June 3 that he would get 'all tests turned around within 24 hours by the end of June, except for difficulties with postal tests or insuperable problems like that'.
Before the new figures were published, Justice Secretary Robert Buckland said the month-long lockdown that began in England on Thursday will be used to 'redouble our efforts' to expand the NHS Test and Trace programme.
Speaking to BBC Breakfast, he said it is also vital to increase the speed at which test results are returned.
'Lots of people are receiving them the next day which is good, but there are still too many people who are having to wait for days and we are going to continue to work to speed that up,' he said.
'We've got to use this time not only to deal with Test and Trace but also to prepare for when we get a vaccine.'
He said any future vaccination programme would prioritise those in greatest need 'so we can avoid a stop and start scenario where we're having to go in and out of lockdowns'.