Just 59.9% of close contacts of people who tested positive for coronavirus were reached by NHS Test and Trace in the week ending 28 October, figures show.
This is the second-lowest weekly percentage for the scheme so far - down from 60.6% in the seven days to 21 October.
Over the same period, 137,180 people tested positive for coronavirus in England, which is up 8% on the previous week.
Where cases were managed by local health teams in England, 97.9% of contacts were able to be reached and told to self-isolate in that week, while cases managed online or by call centres saw a rate of just 58.5%.
The figures cover the same week where thousands of people were given the wrong dates for self-isolation by Test and Trace after a software error.
Only 26.4% of people who were tested in England at a regional, local or mobile site got their result within 24 hours - up slightly from 22.6% in the previous week.
Prime Minister Boris Johnson had previously promised in-person tests would all be returned within a 24-hour timeframe by the end of June.
Speaking in the House of Commons on 3 June, he made the pledge for the end of the month, except for "difficulties with postal tests or insuperable problems like that".
During Prime Minister's Questions, Mr Johnson admitted there had been issues with the service, but that he was proud that it had got up and running so quick.
On Wednesday, the UK reported another 492 coronavirus-related deaths, which is the highest daily total since 19 May, alongside 25,177 positive tests.
On Thursday, England was placed in to lockdown - with non-essential businesses, pubs, restaurant and gyms all closing - as the government tries to slow the rise in coronavirus infections.
Analysis: It's time for Test and Trace to seize the opportunity
By Rowland Manthorpe, technology correspondent
The Test and Trace figures for the last week of October confirm what we already suspected: as cases rose, contact-tracers struggled to cope.
An overall percentage of 59.9% for contacts reached is the lowest figure ever reached by Test and Trace. (Sort of. The lowest figure ever was 59.6%, but the government later changed that number to 60.1%, so today's figure is now the new low. Confused? Welcome to the world of Test and Trace data.)
Government insiders sometimes complain that it's unfair to compare percentages across weeks, because the absolute numbers being reached are higher.
Statistically, they have a point.
But however you cut the numbers, the bottom line is: Test and Trace is still not reaching lots of contacts. That's not good.
It's easy to blame Test and Trace, which has certainly made many mistakes. (Just this week I revealed that it had given more than 7,000 people the wrong dates for self-isolation).
Yet, in truth, the real culprit is the rising number of positive cases.
Between 22 October and 28 October, contact-tracers were asked to reach 327,203 people, an increase of 16% on the previous week.
No system in the world would be able to manage numbers like this on a regular basis.
International comparisons bear this out very clearly.
It's not just that their numbers went down; as they were put under strain, they experienced the kind of embarrassing error we've grown used to in England.
Did you know that Irish contact-tracers recently asked 2,000 people to do their own tracing? Or that Germany "lost" testing data for days after a glitch?
The Test and Trace system definitely has inadequacies, but it is not completely bad, nor are its struggles uniquely unusual. Now it has a month to improve. It needs to seize the opportunity.