Demand for coronavirus testing is “significantly outstripping the capacity we have”, head of NHS Test and Trace Baroness Harding has told MPs.
Lady Harding told the science and technology committee that the return to school meant demand from under-17s had doubled.
She acknowledged that results were also taking “slightly longer”.
But she said she was “very confident” of raising capacity to 500,000 tests a day by the end of October.
“I am certain we will need more as we go beyond the end of October. We have plans to go beyond 500,000 a day,” Lady Harding said, before adding there was no formal target beyond the October deadline.
The test and trace programme has come under increasing pressure in recent days, with reports of people unable to access tests or being directed to test centres many miles away.
Figures published on Thursday also showed the turnaround time for community tests was getting longer. Only a third of these tests came back in 24 hours in the week up to 9 September, compared to two-thirds a week earlier.
It comes as the UK reported another 3,395 confirmed cases of coronavirus, and a further 21 deaths were recorded within 28 days of a positive test.
The number of people calling 119 and visiting the website to book tests was three to four times the number of available tests, Lady Harding told the committee – although she said that may exaggerate the problem as some people call repeatedly from different numbers.
Committee chairman Greg Clark said it was “dispiriting” that despite the “entirely predictable” circumstances of the return to schools and offices “we haven’t had the right capacity put in place”.
Lady Harding said they built the testing capacity for this autumn – which is now 242,817 a day – based on modelling from the Sage scientific advisory group.
“I don’t think anybody was expecting to see the really sizable increase in demand that has happened over the last few weeks,” she said.
In Sunderland, meanwhile, more than 100 people were left waiting at an empty car park where they said they had been booked in for Covid-19 testing, although no staff or equipment was there.
And in Lewisham, south London, people seeking tests said the approach to the centre was “gridlocked” and people were being sent away.
Lady Harding said testing was limited by the laboratory processing capacity, and that they had to restrict the number of people at centres because it would be “very dangerous” to send too many samples to the laboratory that would then go untested.