There were no walk-in PCR tests available to book in England for a few hours today due to "high demand".
Appointments had been unavailable in every region of the country, although there were some in Northern Ireland, Scotland and Wales.
Although people were still able to order a test to be sent to their home.
However, later on in the afternoon, bookings were available in every region, with the UK Health Security Agency saying the reduced availability was merely "temporary".
A UKHSA spokesperson said: "Appointments for PCR tests are available but high demand can lead to temporarily reduced availability in some areas.
"More appointments are made available for people to book every afternoon.
"Testing is playing an integral role in keeping people safe and managing the spread of Covid-19.
"We are very grateful to everyone who is following the Government's guidance and getting tested to protect themselves, their loved ones and their communities.
"Everyone with symptoms or who is directed to get a PCR test should continue to come forward to get a test delivered to their home or at a test site."
A paramedic, named Ethan, from Southampton, said he had been "fuming" over initial fears he could miss work after waking up with Covid symptoms.
He said: "I've experienced symptoms overnight and into this morning of a new cough meeting the Covid guidelines for requiring a PCR test.
"I've tried this morning to book a test locally for which nothing whatsoever was available. I've had to book a testing kit with an estimated delivery of two days and unsure of how long it will take results to return back.
"I'm a NHS paramedic and I was due back into work tomorrow to work through into the New Year.
"My trust is already experiencing high staff sickness rates due to stress and other ailments and Covid-19 related illness, and I'm fearing that this delay in me getting results compared to the length of time it would take me to get a drive-through PCR done and the results from that will negatively affect my colleagues and the public."
Other were concerned the lack of availability could affect travel plans, sports fixtures and business appointments.
