Tens of thousands of people have booked coronavirus tests after the government expanded who can apply in England.
Care home residents and staff, people over 65 and anyone who cannot work at home are now among millions eligible to be tested if they have symptoms.
Some 14,700 home tests and 33,000 drive-through appointments were booked on Wednesday, the government said.
It has repeatedly said it is aiming to reach its target of 100,000 tests a day by Thursday.
The expansion in eligibility beyond just essential workers and hospital patients means 25 million people can now book through the government's test-booking website.
Symptoms of coronavirus include a new dry persistent cough, and a fever or raised temperature above 37.8C.
All 14,700 home test kits made available on Wednesday at 08:00 BST were fully booked in 90 minutes, Downing Street said.
A spokesman added that 33,000 people had booked in for tests at drive-through centres by 09:00 BST, and said that the website is still accepting bookings.
Testing capacity has now increased to 73,400 a day, according to Downing Street, though home testing kits and appointments at regional centres in Wales ran out around an hour after Wednesday's slots launched for booking.
The Department of Health promoted available testing appointments at a number of sites across England and Scotland on Wednesday afternoon, suggesting supply was outpacing demand in some areas.
But differences between the UK nations on testing emerged as Wales said it would not follow England in testing care home residents and staff who are not displaying symptoms.
Who can be tested in England?
- Hospital patients suspected of having Covid-19
- All those working on the frontline in health and social care (with or without symptoms)
- Patients in the NHS and residents in care homes (with or without symptoms)
- All other essential workers with symptoms
- Anyone over 65 with symptoms
- Anyone who goes into work because they cannot work from home (for example, construction workers) and has symptoms
- Anyone who has symptoms and lives with someone who meets any of the above criteria
In the Commons on Wednesday, Foreign Secretary Dominic Raab said Prime Minister Boris Johnson's 250,000 tests a day target remained "an aspiration" in response to questions from Labour Party leader Sir Keir Starmer.
Mr Raab said that, while the goal was still in place, he would not confirm when it would be reached.
"The key point is the 100,000 milestone is only the first stepping stone towards testing, which is essential to the wider testing, tracking and tracing regime we'll need as we transition to the second phase [of combating the epidemic]", he added.
Mr Johnson missed Prime Minister's Questions after he and fiancee Carrie Symonds announced the birth of a healthy baby boy in London but has already returned to work at No 10, his official spokesman said.
The PM held a "constructive" phone call with Sir Keir on Wednesday afternoon, he said, adding that the pair have agreed to continue speaking about the national effort to defeat coronavirus.
Meanwhile, ministers are yet to receive advice from the government's science advisers on easing lockdown measures, No 10 said.
It comes as the government prepares to publish up-to-date figures for coronavirus deaths, including for the first time those in the community, such as in care homes.
Office for National Statistics data released on Wednesday showed a third of all coronavirus deaths in England and Wales are now happening in care homes, based on reporting up to 17 April.
Eligibility criteria for testing varies around the UK.
In Scotland, First Minister Nicola Sturgeon said all patients over 70 who are admitted to hospital for any reason will now be tested for Covid-19.
In Northern Ireland, testing has begun in some GP practices and will also be expanded in hospitals and care homes.
Wales has adopted a needs-based approach to testing for NHS and "critical" non-NHS workers, but Health Minister Vaughn Gething said the country would not roll out tests to care home residents and staff who are not displaying symptoms, unlike England.
In addition to civilian testing, more than 800 sailors have been tested onboard the aircraft carrier HMS Queen Elizabeth, and its departure from dock has been delayed.
'Massive expansion in testing'
This represents a massive expansion of who is eligible for testing - and means we are now one step away from allowing everyone to access a test if they have symptoms.
That will be crucial when lockdown restrictions are eased as part of the "test, track and trace" strategy to keep coronavirus at bay.
It is being made possible by the roll-out of home-testing kits and mobile units staffed by the armed forces.
The problems experienced getting more people tested have - to some extent - been because the network of drive-through testing centres have not always been in convenient locations.
There is plenty of lab capacity to process the tests now the three mega labs are up-and-running in Milton Keynes, Glasgow and Cheshire.
The expansion has also allowed the government to do something that could prove crucial in tackling the epidemic in care homes - the testing of residents and staff without symptoms.
A big concern is that the virus has been able to get a foothold in care homes via people transmitting it before they develop symptoms or if they are asymptomatic.
But promising something is one thing - delivering it is another.
While the capacity looks like it will be there to test 100,000 a day by the end of the month, the numbers actually getting tested are currently less than half that.
In other developments:
- Some 4,516 care homes have reported coronavirus outbreaks since 17 March until Monday this week - around 29% of all such facilities, according to Public Health England
- Downing Street said a cabinet meeting on Thursday would look at the "coronavirus response in general" but is not expected to make decisions on easing lockdown measures
- New figures showed the total number of deaths linked to coronavirus in Scotland has reached 2,272
- It was confirmed more than 1.3 million Britons have been repatriated since the coronavirus outbreak
- Patients have begun receiving medical cannabis through the post, as the coronavirus pandemic has left them unable to access the drug any other way
- Retailer Next said it has plans in place to reopen out-of-town stores once restrictions are lifted as it warned sales will suffer
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