The Government has run out of lateral flow home test kits as people wait to book a coronavirus vaccine.
People wishing to order the test are met with a message saying "there are no more home tests available".
People are advised to "try again later" or to book a test site appointment instead.
One person said on Twitter: "Tried to order a lateral flow test kit and the NHS site says they have no more! What's going on!"
However, there appeared to be no problems with the availability of PCR home test kits.
It comes as people trying to book a Covid-19 booster jab in England have been advised by the NHS to try later or tomorrow as people faced a queue on the website.
In a tweet, the NHS said: "The Covid vaccine booking service is currently facing extremely high demand so is operating a queuing system.
"For users aged 18-29, please be aware that booking opens on Wednesday 15 Dec. For all others experiencing waits, we would advise trying again later today or tomorrow."
It comes on the first day that 30 to 39-year-olds in England can officially book the jab.
The service had already booked more than 140,000 vaccine appointments on Monday, NHS Digital said, with people waiting several minutes.
The PA news agency has approached UK Health Security Agency (UKHSA) for comment.
Boris Johnson announced an accelerated booster rollout plan on Sunday and urged people to book their third dose to tackle the spread of the Omicron strain.
Mr Johnson urged that the strain could rip through an un-boosted population, overwhelming the NHS and leading to "very many deaths".
He said scientists had discovered that two doses of a vaccine is "simply not enough" to prevent the spread of the new variant.
However on Monday Mr Johnson said there was a "ready supply" of lateral flow tests despite what the website said.
The Prime Minister was asked about the supply and told broadcasters: "They can get those tests, we do have a ready supply of lateral flow tests.
"If you can't get one online for any reason, then there are ample supplies in the shops. But what I think, if I may say so, what that also shows is that people are doing the sensible thing, and getting tests as well."
The Government's ambitious programme will aim to jab almost one million people per day for the rest of December.
However nursing leaders and a cancer charity have expressed concerns about the impact this may have on the NHS.
The NHS will postpone some planned appointments in order to meet the target.
NHS England said GP teams will be asked to "clinically prioritise their services to free up maximal capacity" to support the vaccination programme, alongside delivering critical appointments such as cancer, urgent and emergency care.
Nursing leaders have expressed concern about the "scale and pace" of the vaccine programme expansion.
Top medics recommended to ministers that the UK be moved from Level 3 to Level 4 warning that the new strain "adds additional and rapidly increasing risk to the public and healthcare services."
