Coronavirus tests for UK key workers were unavailable through the government booking website within an hour of it reopening, apart from some in Scotland.
The site had to close to new applicants within hours of launching on Friday, after 46,000 people tried to access it. Some 16,000 bookings were made.
Home testing kits became unavailable less than 15 minutes after bookings reopened on Saturday morning.
Tests at drive-through sites in England became unavailable within an hour.
Drive-through tests are still currently available in Scotland.
No 10 has said appointments for tests at drive-through centres and home testing kits would become available each day from 08:00 BST, with their release staggered throughout the day.
Prof Stephen Powis, medical director of NHS England, urged key workers showing Covid-19 symptoms to book a test on the government website.
He told BBC Breakfast there was now capacity for more than 50,000 daily tests – and that reaching 100,000 a day by Thursday remained “the aim”.
“The NHS has committed to capacity of 25,000 within NHS laboratories and we are on trajectory for that capacity to be in place,” he said.
Prof Powis added the test people can book online is a swab test to determine whether people currently have the virus – rather than whether they have had it in the past.
“I would urge people who fit in that criteria, key workers, to go back onto the website as more appointments become available,” he said.