The new website for key workers to book coronavirus tests has temporarily closed - hours after being opened by the government.
The Department of Health said it was working to "rapidly increase" availability, with more tests due to become available on Saturday.
Downing Street said 5,000 home testing kits were ordered in the first two minutes of the website going live.
About 15,000 tests were booked to take place on Friday at drive-through sites.
No 10 said the total amount of home testing kits available on Friday were ordered, adding it hoped to have 18,000 kits available each day for key workers by the "end of next week".
The website would be back up and running when the next batch of slots become available, the spokesman added.
Up to 10 million key workers and their households are now eligible for testing, with the aim of allowing essential staff to safely return to work.
Previously, a smaller group of key workers - those whose work is deemed critical to the Covid-19 response - were eligible for testing.
Scotland's First Minister Nicola Sturgeon said the new system would also apply there "very soon", while Wales and Northern Ireland have both announced separate plans to expand testing.
Downing Street said the government is trusting that those applying for testing are key workers, with no eligibility checks in place for online bookings.
The website opened on Friday but appeared to close by mid-morning with some users met by an "applications closed" message.
The Department of Health apologised for "any inconvenience" caused by the temporary closure of applications.
The government remains some way short of its target of carrying out 100,000 tests a day in the UK by the end of this month.
Friday's figures showed 28,532 tests were carried out, though Mr Hancock said on Thursday that capacity has reached more than 50,000 a day.
The latest data also showed a further 684 people died with the virus in UK hospitals, bringing the total number of deaths to 19,506.
In Wales the total number of people confirmed to have died with coronavirus reached 751, an increase of more than 100 for the first time. However, Public Health Wales said the 110 deaths reported on Friday included 84 that had been added retrospectively "due to a delay in the reporting process".
A mixed picture
Secondary school teacher Mike Eyre, from Boston in Lincolnshire, was one of the first people to apply for a test but told the BBC he was still waiting for confirmation about where to go 11 hours later.
"It was very easy to log on and register," he said. "It just said that you would then receive a text, which would take a few minutes but it might take a little bit longer if it's busy.
"I've still not had a text yet, so I'm assuming they're very busy."
Liz Christie, an education adviser from Slough in Berkshire, applied at about 06:45 BST and heard back at about midday on Friday.
She will make the 30-mile round-trip to a drive-in centre in Wembley, north London, later.
She told BBC News that she "didn't expect it to be completely smooth" on the first day, and said she was "pleased that in less than 48 hours I'll have been tested and know if I'm positive or not".
"We've been able to work from home but they've asked if I can go in next week, so it would be useful to know," she said.
"My children are both asthmatic - and getting a test would also mean I could then go and help my mum, who has almost no immune system and is getting no help in the house."
John Apter, chairman of the Police Federation of England and Wales, described the announcement as "long overdue and very welcome".
He added that testing would enable self-isolating officers "to get back onto the front line to assist their colleagues if the test comes back as negative".
Also among those to qualify for tests are staff at supermarkets, where absences due to coronavirus have been a big issue. Tesco currently has more than 41,000 staff members off work.
The British Retail Consortium said the testing of such workers was "crucial" to "maintain good supplies to stores".
Transport for London (TfL) - where a third of employees have been ill or have had to self-isolate since the start of the pandemic - said that while testing would not be mandatory, it was "encouraging the take-up of testing by all eligible members of staff".
Mr Hancock said once people entered their details online they would be sent a text or email inviting them to either book an appointment at a drive-through centre or to request a home test kit, though the latter are currently in limited supply.
He told BBC Breakfast the home kits would be collected by courier and would initially start in "small numbers" before the service grows.
A help desk has been established to aid the process, while mobile units run by the Army are travelling around the country to where they are needed, such as care homes.
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.
Mr Hancock has said the "majority" of people will get their result in 24 hours, but for some it might take a bit longer.
The result will be added to a person's medical record so their doctor is informed, he added.
Key workers who are unable to access the government's website will still be able to apply for a test, as employers are able to book on behalf of their staff.
Until now, hospitals have been carrying out tests, along with a network of about 30 drive-through centres in car parks, airports and sports grounds.
Labour's shadow health secretary, Jonathan Ashworth, said ministers should have been prepared for the surge in demand for coronavirus tests.
There is no doubting the scale and ambition of this project - up to 10 million essential workers and their families invited to go online if they have symptoms to book a virus test.
Ensuring the system can smoothly book people their tests at drive in centres and then get results back within 48 hours will be a major technological challenge.
Some may complain that the drive-in centres are not accessible but the plan will soon involve home testing kits being sent to people to do their own swabs and send them back for laboratory testing.
The home testing kits are currently going out to NHS workers. Then government says this will be extended very soon to the wider group.
Mobile pop-up test facilities will go to workplaces.
In theory this will make a big difference to key workers self-isolating because a member of their household is unwell - if that individual tests negative then others in the home can get back to work quickly.
Or if someone is unsure about their symptoms they can easily check whether they have coronavirus.
This is a significant move in the drive to boost testing.
The question now is whether the IT and the infrastructure can deliver.
At Thursday's Downing Street briefing, the health secretary also pledged to recruit 18,000 people to trace those who had been in contact with those infected.
Meanwhile, Prof John Newton, co-ordinator of the UK's coronavirus testing programme, said the government is "on track" to reach 100,000 tests a day by the end of April.
In other developments:
- Dyson said the medical ventilator it developed to help treat patients with Covid-19 is now no longer required
- About 7,000 Transport for London (TfL) staff are to be furloughed amid "massive financial challenges" for the transport authority
- Some lockdown rules in Wales are being made stricter to "address challenges being faced in parts of the country", the first minister has said, including a clamp down on the use of second homes
- Baroness Doreen Lawrence has been appointed as the Labour party's race relation adviser and will lead a review into the impact of coronavirus on black, Asian and minority ethnic communities amid concerns the virus may be disproportionately impacting them
- Gatwick Airport has said it expects it will take up to four years for passenger numbers to return to recent levels once the coronavirus pandemic recedes
- US President Trump has said Prime Minister Boris Johnson sounded "sharp and energetic" in a phone call on Tuesday, meanwhile Mr Hancock said he was "on the mend" but his return date to work is a matter for his doctors
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