People are waiting longer for coronavirus test results from England's community testing centres, figures show.
Only a third of tests carried out in these venues came back in 24 hours in the week up to 9 September.
That is down from two-thirds the week before, NHS Test and Trace said.
Health Secretary Matt Hancock admitted there was "a challenge in testing" as he faced criticism in the Commons.
He also announced new lockdown restrictions for most of the north east of England, where infections are on the rise.
Access to community testing has had to be rationed because labs are struggling to keep up with demand, but this is the first evidence tests which do happen are taking longer to process.
There are three types of community testing centres - drive-throughs, walk-in centres and mobile units that are deployed to hotspot areas.
All three saw rises in turnaround times.
- Average turnaround times for regional drive-through centres rose from 20 to 27 hours, with 38% returned in 24 hours
- For local walk-in centres the average was 35 hours, with just one in five results delivered in 24 hours
- Mobile units faired best with an average of 26 hours, up from 19 the week before. Some 38% of results were given in 24 hours.
Over the week, 360,000 tests were carried out in these three settings, up from around 320,000 the week before.
The release of the turnaround times comes as growing numbers of people complain they cannot access tests at all.
Booking slots at testing sites, as well as the availability of kits that are posted out to people's homes, have been restricted across the UK because labs are not able to keep up with demand.
It has meant tests have had to be prioritised for high-risk areas, including care homes and areas where there are local outbreaks.
Shadow Health Secretary Jon Ashworth said the government must urgently fix test and trace to "avoid further restrictions", adding: "It's become not so much test and trace, more like trace a test."
And a succession of MPs raised the cases of constituents who had struggled to access tests.
Mr Hancock admitted there was "a challenge in testing", but said capacity was "at record levels" and had increased week on week but demand had "gone up faster".
He said it was important to prioritise testing in areas where cases are rising, as well as in care homes, saying "we must do everything in our power to protect residents in social care".
The government has announced care homes in England will receive extra funding of £546m to try to reduce transmission of coronavirus this winter.
'Boost testing capacity'
Experts are warning that testing problems will limit the UK's ability to contain spread of the virus.
Hospital labs, which process tests for patients and NHS staff, are not affected by the problems. Nearly nine in 10 tests there are turned around in 24 hours.
The government said testing capacity would be increasing. Currently 375,000 tests a day can be processed - although only around 160,000 of these are in the labs that process community tests.
For days software developers have been discussing what they described as a coding error on the government's Covid test booking site.
Now a senior source has confirmed that there is such an error although it is described as small and not a major factor in the issues with booking tests.
The problem, says the source, is that when the site is overwhelmed by the sheer volume of traffic - and "the load this week has been way higher than anything seen before" - users are getting the wrong error message.
It should read "the service is busy" but instead it says "there are no sites available".
Now, the reality is that mostly that is true, there are no testing sites free - but some people who might be successful if they tried later when the site traffic was lower may assume they can't get a test at all.
Two new labs are due to open soon, which would bring overall capacity to 500,000 by the end of October, with another two planned for early in 2021, the government said.
But NHS Test and Trace boss Baroness Dido Harding said it was important that "only those with symptoms book tests".
She added: "The service is there for those experiencing a high temperature, new continuous cough or loss or change in sense of taste or smell.
"If you don't have symptoms but think, or have been told by NHS Test and Trace that you have been in contact with someone with the virus, please stay at home but do not book a test.
"We need everyone to help make sure that tests are there for people with symptoms who need them."