NHS Test and Trace has recorded its worst ever week for contact tracing as cases of coronavirus continue to rise.

It is understood senior Government officials have expressed concern that Test and Trace could become “overwhelmed” if case numbers get too high.

They argue Test and Trace works best when incidence of Covid-19 is at lower levels and smaller outbreaks can be stopped in their tracks.

But they said it is difficult for the system to have an impact in places where incidence of coronavirus is high.

Test and Trace is also having less impact than hoped due to the fact some people who test positive find it very difficult to isolate.

New data shows 62.6% of close contacts of people who tested positive for Covid-19 in England were reached through the system in the week ending October 7.

This is the lowest weekly percentage since Test and Trace began, and is down from 69.5% the previous week.

For cases handled by local health protection teams, 97.7% of contacts were reached and asked to self-isolate in the week to October 7.

For those handled either online or by call centres, 57.6% of close contacts were reached and asked to self-isolate.

The way the system operates has been the subject of a political storm, with Labour highly critical of the way private firms have been used.

Shadow health secretary Jonathan Ashworth highlighted reports that Boston Consulting Group executives had been paid “over £6,000 a day to run this failing service”.

“In a single week this Government is paying these senior consultants more than they pay an experienced nurse in a year,” Mr Ashworth said as he questioned why “such huge sums of money are being paid to consultants to run a service that is only getting worse”.

The Opposition has also called on the Government to ditch outsourcing giant Serco from the £12 billion operation.

The latest data shows 89,874 new people tested positive for Covid-19 in England in the week to October 7 – a 64% rise in positive cases on the previous seven days.

It is the highest weekly number since Test and Trace was launched at the end of May.

When it comes to turnaround times for test results, there has been a slight week-on-week improvement.

Some 32.6% of people who were tested for Covid-19 in England in the week ending October 7 at a regional site, local site or mobile testing unit – an in-person test – received their result within 24 hours.

This is up from 27.4% in the previous week.

Prime Minister Boris Johnson pledged earlier in the year that by the end of June, the results of all in-person tests would be back within 24 hours.

Of the 87,918 people transferred to the Test and Trace system in the week to October 7, 76.8% were reached and asked to provide details of recent close contacts.

This is up slightly on 74.9% in the previous week.

This week’s figures include around 2% of the almost 16,000 cases that were previously unreported following technical errors at Public Health England.

Health Secretary Matt Hancock defended the system, telling MPs: “Through NHS Test and Trace we’ve built up a detailed picture of where and how this virus is spreading.

“This week’s NHS test and trace statistics show the testing capacity is up, testing turnaround times are down, and the distance travelled for tests is down too.”

But NHS Providers deputy chief executive Saffron Cordery said: “It’s deeply unfortunate that at this point with infections rising, admissions increasing and winter looming, there’s still clearly a long way to go until our test and trace system is fit for purpose.”