A message on the website said: “The NHS website is currently experiencing technical difficulties.
“We are working to resolve these issues. Thank you for your patience.”
Other users reported being placed in a queue, with a holding screen which read: “You are in a queue. Lots of people trying to book an appointment.”
The difficulties arose just as over-45s were invited to book their first dose of the Covid-19 vaccine on Tuesday, as all over-50s have been offered a jab ahead of schedule.
This marks the second phase of the vaccination rollout, which involves offering vaccines to healthy adults under 50.
Hailing the “significant milestone,” Prime Minister Boris Johnson said: “More than 32 million people have been given the precious protection vaccines provide against Covid-19.
“We will now move forward with completing essential second doses and making progress towards our target of offering all adults a vaccine by the end of July.”
According to the latest figures available, over 32 million people in the UK have now received a first dose of a Covid-19 vaccine while over 7.6 million have received both jabs since the rollout started in December 2020.