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.
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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.