NHS England said nearly 500,000 appointments had been reserved by midday on Tuesday (8 June) - mere hours after eligibility was widened to the over 25s age group.
It equates to around 100,000 bookings an hour, and the number is more than double the appointments booked the previous day.
‘Glastonbury-style rush’ for appointments
NHS England chief executive Sir Simon Stevens said: “This encouraging Glastonbury-style rush for appointments has already now seen hundreds of thousands of people between 25 and 29 book in for their NHS Covid jabs, as more vaccine supplies continue to come on line.
“Pleasingly this suggests strong enthusiasm for vaccination amongst people in their 20s, following hard on the heels of the millions of others who’ve already taken up our offer.”
The surge comes on the six-month anniversary of grandmother Margaret Keenan, 91, becoming the first patient in the world to receive a Covid-19 jab outside a clinical trial when she was given the Pfizer/BioNTech vaccine in Coventry on December 8.
Many young people reported problems with the booking website on Tuesday morning, with some posting screenshots to social media, showing an error message telling them that they are “not currently eligible to book through this service”.
It directs them to call the 119 helpline to try to book a vaccination.
Others reported being told they were in a queue of thousands.
Some received ‘ineligible message’ when attempting to book
A spokesperson for NHS Digital said: “Large numbers of people are currently booking their vaccine appointments through the NHS website, which means you may need to wait in a queue.
“We know that some people have been receiving an ineligible message when trying to book, which is being fixed now, so please retry.”
The vaccine booking website has suffered glitches in the past, notably crashing in April after appointments were being opened up to the over-45s.
Last month, an apparent security flaw was also uncovered in the website which could allow anyone to work out another person’s vaccination status by entering their basic personal information – name, date of birth and postcode.
Privacy experts warned the flaw was exposing personal health information which could be exploited by scammers.
At the time, NHS Digital said it was reviewing and improving the standard messages that were presented on the website.