Boris Johnson’s hopes of a “moonshot” mass testing for Covid-19 have suffered a fresh blow after his flagship NHS Test and Trace programme posted its worst ever figures for contact tracing.

The PM’s independent scientific advisers have repeatedly warned that stopping the spread of the disease requires more than 80% of “close contacts” of people with Covid reached by the system to be identified and then told to self-isolate.

But figures for the latest weekly update show that just 69.2% of those “close contacts” – defined as being within two metres of a Covid carrier for more than 15 minutes – are actually being reached.

The statistics, for the week between August 28 and September 2 show a fall on the 69.4% of the previous week – and are the lowest weekly percentage since the service launched in May.

Testing turnaround times are also well short of the 100% within 24 hours target set by Johnson, with an average of just under 63% of in-person tests meeting that deadline. Times for home testing and ‘satellite’ care home testing fell again.

In line with the spike in cases reported in recent days, the figures for the week to September 2 showed a huge jump in positive tests, with a 43% increase week-on-week.

On Wednesday, the prime minister pinned his hopes for a return to normality in the UK on a “moonshot” plan to get millions of instant tests to the public for daily updates on their Covid status.

Under his proposal, which has been met with heavy scepticism by even his chief scientific adviser, those who test negative would then be free to go to work, the theatre or sports events.

With the public facing lengthy delays and an acute shortage of local testing availability, critics have suggested the government focuses its time and money instead on improving NHS Test and Trace.

Many people who tried to access a test on Wednesday were met with an error message telling them to try again, and warning them not call the helplines.

And on Tuesday, Sarah Jane Marsh, the service’s director of testing, apologised to people who were unable to get a test.

Johnson, who has described the service as “world beating”, this week used a different definition for the close contact rate, preferring to say that where contact details were provided it was actually at 80%.

Today’s figures showed that even on that definition, the percentage had dropped to 79.4% from 80% the week before.

Government insiders say that the low contact trace figures are explained partly by the shift from hospital and care home cases – termed “complex cases” –  towards community transmission, where it is more difficult to identify and trace people.

Ministers hope that the new compulsory requirement on pubs and restaurants to keep lists of contacts of customers will help improve the figures. Under the PM’s latest tightening of restrictions, hospitality venues will be legally required to keep lists for 21 days.

But shadow health minister Justin Madders said: “There is nothing world beating about this test and trace system.

“Taking into account all contacts identified, 69.2% were reached not the 80% claimed by the Prime Minister yesterday. The percentage reached for both complex and non-complex contacts has stayed roughly the same since mid-July.

“With millions of people having returned to work and schools this week it is concerning that the system is not improving. This continuing failure does nothing to provide the public with the safety and assurance they need.

“In the week where the testing system failed to provide locally available tests for people who desperately need them, it seems nobody in Government has yet picked up responsibility to turn things round and for the Prime Minister to not even know how the system is performing is a worrying sign of the lack of grip Government has on the scale of the problem.”

NHS Test and Trace chief Dido Harding had not made a public comment for a week. One Whitehall source told HuffPost UK: “Is it a case of ’disappearing Dido? This week of all weeks, test and trace is in the news.”

But Harding did issue a statement on Thursday lunchtime, insisting that the service was “working” and that the UK was doing more testing than other comparable European countries.

“The figures show we have seen a significant increase in the demand for tests, but given the concerning rise in cases over the last couple of days, it is still vital for anybody who has symptoms to book a test and follow the advice you receive if you are contacted by NHS Test and Trace,” she said.

“New booking slots and home testing kits are made available daily. And for those who don’t have symptoms or haven’t been told they must have a test, we would please ask you to reconsider as it might could be taking a test away from someone who really needs it.”

And business chiefs waded in too, with London Chamber of Commerce chief Richard Burge urging the government to “instil confidence in the integrity of the system immediately”.

“A fully functioning Test and Trace system is imperative for managing the spread of Covid19 and saving lives.

“But it is also vital to economic recovery, whether by instilling consumer confidence in areas of high population such as London, or avoiding businesses having to scale-back or shutdown due to reintroduced restrictions – whether locally or nationally.”

There was better news for the government on turnaround times for tests, with a rise in the percentage of in-person tests (at mobile and regional test centres) results received within 24 hours.

The figure went up to 61.9% compared to 53.3% in the previous week – but it is a long way short of the 100% that Johnson set as a target by the end of June.

He told the House of Commons on June 3 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”.

The department of health and social care said that on a different measure – of getting test results by the end of the following day, rather than within 24 hours, 89.6% of in-person tests results were received “the next day after the test was taken”.

But some turnaround times worsened again, with 10.7% of test results were received within 48 hours for home test kits and satellite test centres (used for care homes and others), compared to 11.3% in the previous week.

Satellite test centres – which are supposed to provide kits for care homes “that have a particularly urgent need” – saw the median time for a result increase from a low of 28 hours in mid-June to 83 hours in the latest week.

A total of 9,864 new people tested positive for Covid-19 in England in the week to September 2, according to the latest figures.

This is an increase of 43% in positive cases on the previous week, and is the highest weekly number since Test and Trace was launched at the end of May.

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