Earlier on Wednesday, UKHSA warned that coronavirus tests could be temporarily unavailable to order due to "exceptionally high demand".

Demand is surging as the more transmissible Omicron variant pushes cases numbers to record levels - and at points on Wednesday morning, lateral flow and PCR tests were both unavailable for home delivery across the UK via the government website.

COVID news live - latest from the UK and around the world

PCR walk-in and drive-through sites were also severely restricted. None were available in England or Northern Ireland, and "very few" in Scotland.

Both tests were available for delivery again shortly after noon, while in-person PCR slots became bookable across the UK. But, after about half an hour of availability, lateral flow tests were again offline.

However, in a statement later on Wednesday, a UKHSA spokesperson confirmed "record numbers" of lateral flow tests are being delivered to pharmacies.

"The UK's testing programme is the biggest in Europe with almost 400 million tests carried out since the start of the pandemic," the spokesperson said.

"We are delivering record numbers of lateral flow tests to pharmacies across the country, with almost 8 million test kits being made available to pharmacies between today and New Year's Eve.

"We have made 100,000 more PCR booking slots available per day since mid-December and we are continuing to rapidly expand capacity - with over half a million tests carried out on 23 December alone and delivery capacity doubled to 900,000 PCR and LFD test kits a day.

"If you have not been able to get the test you need from gov.uk, please keep checking every few hours as more PCR and LFD tests become available every day."

Pharmacies have already been reporting very high demand, with people coming in "every five minutes" for tests, according to an industry representative.

Work and Pensions minister Chloe Smith told the BBC the government was "working extremely hard to make sure that [testing] capacity is there".

Demand is also believed to have been driven up by recently updated COVID isolation rules.

People can now end isolation after a week instead of 10 days if they get two negative lateral flow tests 24 hours apart - the first taken on the sixth day.

Close contacts who are fully vaccinated also do not need to isolate but are strongly advised to take a lateral flow test every day for a week.

Large numbers of absences are badly affecting the NHS, with four in 10 staff in London currently not at work due to sickness or coronavirus quarantine.

It has led to calls to follow the US and cut isolation to five days to protect key industries.

Despite the high number of cases in the UK, the number seriously ill on a hospital ventilator has so far not significantly increased, apparently backing up studies suggesting Omicron is not as harmful as the Delta variant.

Boris Johnson said on Wednesday that "up to 90%" of people currently in intensive care with COVID have not had their booster jab.