Coronavirus tests for UK key workers were booked up through the government 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 were booked up within an hour.
Requests for drive-through tests in Scotland are still currently available on the site.
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.
Dr Simon Eccles, chief clinical information officer at NHS Digital, said an “amazing team” had worked “all night” to enable the site to reopen on Saturday.
After home testing kits were booked up, he wrote on Twitter: “I know it’s frustrating but we’re developing more lab, supply and logistics capacity every day.
“If we’d waited until we had the full 100k, to launch, no-one would have had a test today.”
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.
Can I get a test?
Anyone classified as an “essential worker” who is showing symptoms can request a test in England.
The list includes NHS and social care staff, teachers, police officers and transport workers.
They and their family can also request a test if someone in their household shows symptoms.
How will I be tested?
The test involves taking a swab of the nose and the back of the throat.
There are two ways to get a test: at a testing site, or with a home testing kit.
Home kits will initially be limited but are being sent to NHS staff.
Most people will get their test results by text within two days.
When can I go back to work?
Provided you and/or those in your household have not tested positive, you can go back to work.
That is so long as you are well enough and have not had a high temperature for 48 hours.
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