More than 10 million key workers and their households are now eligible for Covid-19 tests as officials race to hit their 100,000-a-day testing target by next Thursday.
However, home testing kits were listed as "unavailable" on the Government's booking website just 15 minutes after it reopened on Saturday morning, according to the BBC.
It was also not possible to book tests at drive-through regional sites in England, Wales and Northern Ireland by 10am.
According to the site, tests at a drive-through regional site in Scotland are still available.
A spokesman for the Department of Health and Social Care said more will be made available from Sunday morning at 8am.
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Some 46,000 people tried to book a coronavirus test on Friday, but, within two minutes of the website going live at 6am, all 5,000 tests for people to carry out at home had been booked.
Meanwhile, more than 15,000 appointments for tests at drive-through centres were also taken quickly, forcing the Department of Health to apologise on Twitter "for any inconvenience".
NHS England's medical director, Professor Stephen Powis, urged key workers showing Covid-19 symptoms to book a test.
Asked how many were available on Saturday, he told BBC Breakfast: "We are going at capacity, over 50,000 now. The aim is to get to 100,000 by Thursday.
"The NHS has committed to capacity of 25,000 within NHS laboratories and we are on trajectory for that capacity to be in place."
Under the expansion of the testing, NHS and social care staff, police officers, teachers, social workers, undertakers, journalists and those who work in supermarkets and food production are among those now eligible.
Test booking slots or home testing kits will become available from 8am each day, the Department of Health has said, with their release staggered throughout the day.
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