The online website for key workers to book coronavirus tests has temporarily closed following “significant demand”, the Department of Health has said.
It said it was continuing to “rapidly increase” availability and more tests would be available on Saturday.
The website opened on Friday but appeared to close by mid-morning with some users met by an “applications closed” message.
Up to 10 million key workers and their households are eligible to book a test.
The move allows all essential workers in England to register for tests on the government’s website, if they or a family member have virus symptoms.
Health Secretary Matt Hancock said the expanded testing programme was “part of getting Britain back on her feet”.
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 100,000 tests a day in the UK by the end of this month.
Thursday’s figures showed 23,560 tests were carried out, though Mr Hancock said capacity had now increased to 51,000 per day.
The data also showed a further 616 people died with the virus in UK hospitals, bringing the total number of deaths to 18,738.
The health secretary announced the new testing arrangements at Thursday’s Downing Street coronavirus briefing, along with a commitment to recruit 18,000 people to trace contacts of those infected.
Those who can now register for a test include NHS and social care workers, police officers, teachers, those in the justice system, supermarket and food production workers, journalists and transport workers.
Previously, a smaller group of key workers – those whose work is deemed critical to the Covid-19 response – were eligible for testing.
The aim is to allow essential staff to safely return to work.
John Apter, chair 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, for example, 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”.
Paddy Lillis, general secretary of the union USDAW, said it welcomed the news that its members – most of whom work in the food and pharmaceutical supply chains – were recognised as key workers and eligible for the expanding testing scheme.
Employers should ensure the testing and results process is “fair and transparent” for workers, and support those who are ill, self-isolating or shielding on full pay, he added.
Mr Hancock said once people enter their details online they will 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.
The test involves taking a swab of the nose and the back of the throat.
Can I get a test?
Anyone classified as an “essential worker” who is showing symptoms can request a test in England and Scotland.
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 minister, Justin Madders, told the BBC he welcomed the move to expand testing to key workers but questioned whether anything was being done to improve ease of access to the 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.
The Welsh government has previously outlined plans to expand testing to key workers and Northern Ireland’s health minister has announced the nation’s testing programme is being expanded to include front-line workers in the private sector.
Scotland is prioritising tests for NHS staff.
Scotland’s First Minister Nicola Sturgeon has said England’s expanded testing system will apply to Scotland, adding she hoped it would be in place “very soon” but there were still a couple of practical issues to be solved.
Mr Hancock also detailed plans for a network of contact tracers that will be used when the lockdown is eased, insisting a process of “test, track and trace” would be “vital” to stop a second peak of the virus.
The hope is that regional outbreaks of the virus can be kept under control by isolating people with the virus, and then tracing their contacts and isolating them.
The health secretary said infrastructure would be put in place so that contact tracing can be rolled out on a “large scale”.