Brits wanting to test for Covid are becoming frustrated over struggles to keep up with the surge in demand for the third day in a row.
Official Government websites tell users that Lateral Flow Tests and PCR are currently "not available".
It comes on the day of the Covid Pass roll out, which will make it mandatory for members of the public to present a vaccine passport to enter some venues.
And for the third day people were forced to stand in snaking queues outside vaccination centres while waiting for their third booster dose.
Speaking of the shortages, UK Health Security Agency chief Dr Jenny Harries said the spike in test seeking has caused "temporary pressure".
Dr Jenny Harries said the NHS is experiencing "unprecedented demand" for PCR tests.
She went on: "We had already planned to increase capacity over the winter, because we predicted a difficult winter. We clearly couldn't predict a variant like Omicron. We have some flexibility there.
"We currently have about 650,000 tests per day available. If you go back to when we had no tests, or very few, about 1,000 I think going back to autumn last year, this is a huge achievement."
She added: "Yesterday the public may have seen some pressure on some of those sites. I'm in Wales. Some of the test sites here will be at seven per cent or eight per cent capacity. In London, they were quite full.
"People can still order tests at home, and we have put in mobile testing units to try and boost those supplies so people can get tested.
"So we do have sufficient tests both for PCR and LFTs (lateral flow tests). But the very rapid rises is causing some temporary pressure."
On Monday, Brits rushed out in their thousands to get a booster jab in a bid to save Christmas as Omicron sparked fears of another locked down festive period.
Since then, members of the public were spotted queuing for hours outside vaccine centres.
But as people rushed to do the right thing in the battle against the mutation drive, the NHS booking website crashed.
Those arriving at vaccination centres faced five-hour long queues and online lateral flow tests ran out.
Last week Boris Johnson announced his "Plan B" to deal with the sharp increase in Omicron cases.
Speaking at a Downing Street press conference, the Prime Minister said the doubling time of the new strain could be "between two and three days" - meaning its spread will be rapid.
Under the stricter rules people must show a Covid vaccine passport to enter some venues in England.
But if someone is unjabbed they can still enter as long as they produce a negative lateral flow test.
Covid passports will be made mandatory for nightclubs and large events, and rules on face masks would be “further extended” from Friday, to “most public indoor venues” including theatres and cinemas.
Covid testing could also see up to one million Brits spending Christmas Day in isolation this year.
Official models suggest that four times as many people could be self-isolating on December 25 compared to last year.
