Three in 10 people testing positive for Covid-19 and identified by NHS Test and Trace could not be contacted, data shows.
New figures from NHS Test and Trace show a 10% drop in one week in the percentage of people transferred to the contact tracing system who are then reached.
Between September 17 and 23, 71.3% were reached and asked to provide information about their contacts.
This is a fall from 80.8% in the previous week.
It comes as other data from the programme shows a rise in the proportion of people who have an “in-person” Covid-19 test getting their result within 24 hours.
Some 38.1% of people who were tested for Covid-19 in England in the week ending September 23 at a regional site, local site or mobile testing unit – a so-called “in-person” test – received their result within this timeframe.
This is up from 28.2% in the previous week and follows weeks of problems with people accessing tests.
The best performance since the programme began was in the week ending July 1, when 94.3% of in-person test results were returned in 24 hours.
Just 2.9% of people in England who used a home test kit for Covid-19 received their result within 24 hours, the latest figures also show, up slightly from 1.8% in the previous week.
Some 30.4% of people received the result of a home test within 48 hours, up from 11.3% 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.
Thursday’s data also showed that 71.6% of close contacts of people who tested positive for Covid-19 were reached through the Test and Trace system.
This is down from 76.3% in the previous week, but above the 69.9% for the week to September 2, which is the lowest weekly figure to date.
For cases handled by local health protection teams, 97.6% of contacts were reached and asked to self-isolate, whereas the figure was 64.3% for cases handled either online or by call centres.
A total of 31,373 people tested positive for Covid-19 in England in the week to September 23, the figures also showed.
This is an increase of 61% in positive cases on the previous week and is the highest weekly number since Test and Trace was launched at the end of May.
Saffron Cordery, deputy chief executive of NHS Providers, said: “We continue to be extremely concerned about the spread of Covid-19.
“The data shows that there has been 61% more people testing positive for the virus than last week.
“We are equally troubled that only 71% of people who were transferred to the Test and Trace system were reached, which is a big decrease from 81% last week.
“While Test and Trace has been built from scratch at great speed, it is becoming increasingly urgent that it does the job we need it to do as we approach winter.
“Trust leaders across the country are expressing increasing concern that current testing shortages are starting to impact on NHS service recovery and winter preparations.”
She said there were still “no clear” plans on routine testing for NHS staff.
Shadow health minister, Justin Madders, said: “For the proportion of people being contacted to drop by nearly 10% in a week is appalling. It should not be happening at this point.
“While some areas have improved, we are still a million miles away from the promise made by Boris Johnson back in June that the majority of people would have their test results back within 24 hours.
“And on the day it is revealed Deloitte, who are contracted by the Government to run Test and Trace, are trying to sell their contact tracing services to local councils, it is clearer than ever that their time would be better spent improving the huge issues in the existing system.”
The Government said testing capacity was increasing and the median distance travelled by people who go for a test is now 4.3 miles, down from 5.2 miles the previous week.
Baroness Dido Harding, interim executive chairwoman of the National Institute for Health Protection, said: “One in eight people in England have now received a test at least once and with nearly 500 testing sites across the UK, the median distance to a test site is now just 4.3 miles.
“These are achievements in which everyone involved can be immensely proud.
“We are continuing to work tirelessly to increase capacity and make tests as accessible as possible.
“Our new lab partnership with Birmingham University is another example of how the public, private and academic sectors are working together to meet our target of 500,000 test capacity a day.
“In the meantime, NHS Test and Trace has now reached more than half a million people and advised them to self-isolate – and in the past week more than 14 million people have downloaded the new Covid-19 app, all of which will help break the chains of transmission.”