Lateral flow and PCR tests are both currently unavailable for home delivery in England and Northern Ireland via the government website.
Demand is surging as the more transmissible Omicron variant pushes cases numbers to record levels.
The UK Health Security Agency has warned “exceptionally high demand” could make tests temporarily unavailable – and that is the case today.
Gov.uk says there are no home delivery slots left for lateral flow tests “right now” but that people might still be able to collect a kit from a pharmacy.
However, pharmacies are also reporting very high demand, with people coming in “every five minutes” for tests according to an industry representative.
PCR tests for the public and essential workers such as NHS staff are also both showing as unavailable for home delivery.
It is a similar picture for slots at PCR walk-in and drive-through sites.
None are available in England or Northern Ireland and “very few” in Scotland. However, there was availability across Wales.
The website advises people to “try again later” if they cannot currently get a PCR test.
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 for the isolation period to be cut again to five days to protect key industries, following a similar move in America.
Mirroring the picture in a number of European countries, coronavirus case numbers hit another new record in England on Tuesday, with 117,093 reported.
However, the number seriously ill on a ventilator in hospital has so far not increased, apparently backing up studies that suggest 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.