The National Pandemic Flu Service website crashed on the day it was launched after the site recorded more than 9.3m hits per hour.
The Department of Health said demand was “unprecedented” with 2,600 hits a second and that it believed many people were visiting the site out of curiosity.
Chief medical officer Sir Liam Donaldson told the BBC: “We have quadrupled capacity but we’re not expecting people to remain curious for very long”.
The site became unavailable shortly after its launch at 3pm yesterday and was then available intermittently throughout the rest of the day.
Visitors to the site were presented with the message: "The service is currently very busy and cannot deal with your request at this time. Please try again in a little while."
The message was displayed shortly after launch and was also showing close to midnight.
The National Pandemic Flu Service was criticised by the Conservative Party who said the web and telephone advice service should have been set up earlier. The Flu Line was originally supposed to be available at World Health Organisation pandemic alert phase four or five, reached in April, but faced delays as EHI Pirmary Care revealed.
The 0800 telephone number for the service has also been criticised after The Times claimed that Vodafone users would have to pay up to 20p a minute to access the service.
However, a spokesperson for Vodafone told EHI Primary Care that Vodafone users would not be charged for calling the National Pandemic Flu Service.
She said she could not confirm as yet whether calls had been zero-rated since the launch of the service yesterday afternoon but that as from today callers would not be charged.