Around one in eight people transferred to the Test and Trace system after testing positive for Covid-19 were not reached in the latest week, new figures show.
It is the largest proportion not reached since the end of last year, and comes as the number of people testing positive rose to its highest total for nearly five months.
The figure has not been this high since the week ending December 30, during the peak of the second wave of coronavirus, when it stood at 12.6%.
Anybody in England who tests positive for Covid-19, either through a rapid (LFD) test or a PCR test processed in a laboratory, is transferred to Test and Trace so their contacts can be traced and alerted.
Some 87.9% of these people were reached in the latest week.
A total of 135,685 people tested positive for Covid-19 in England at least once in the week to June 30.
This is up 71% on the previous week, and is the highest number of people testing positive since the week to February 3.
The sharp jump in positive cases reflects the impact of the third wave of coronavirus that is continuing to spread across the country.
The latest data suggests the Test and Trace system is facing pressures similar to those seen during the second wave of the virus last winter.
Just over three-quarters of people (76.9%) who were tested for Covid-19 in England in the week ending June 30 at a regional site, local site or mobile testing unit – a so-called “in-person” test – received their result within 24 hours.
This is down from 83.8% in the previous week and is the lowest percentage since the week to January 20.
Prime Minister Boris Johnson last year pledged that, by the end of June 2020, the results of all in-person tests would be back within 24 hours.
He told the House of Commons on June 3 2020 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”.
Separate figures also released on Thursday show that Covid-19 case rates in all regions of England are now at their highest level since at least February.
North-east England is recording the highest rate, with 613.4 cases per 100,000 people in the seven days to July 4, according to the latest surveillance report from Public Health England
This is the highest rate for the region since comparable figures began in summer 2020, when mass testing was first introduced across the country.
All other regions are recording their highest rate since late January or early February this year, except for Yorkshire and the Humber where the rate is the highest since mid-November last year.
Case rates are also continuing to rise for all age groups, with 20 to 29-year-olds recording the highest rate of 614.3 cases per 100,000 people, Public Health England said.
It is the highest rate for this age group since the week to January 17.
Both five to nine-year-olds (248.6 cases per 100,000) and 10 to 19-year-olds (578.6) are now recording their highest rates since comparable figures began.