A new website to diagnose people with swine flu is experiencing "unprecedented demand", the Government has said.
The system was receiving 2,600 hits per second - or 9.3 million hits per hour - at around 5pm.
The website crashed within minutes of launching but appeared to be running normally a short time later.
However, the Government admitted it was having to increase capacity due to demand.
It comes after new figures showed there were an estimated 100,000 new cases of swine flu last week - around double the 55,000 in the previous week.
A spokeswoman for the Department of Health insisted that the website for the National Pandemic Flu Service "has not crashed".
She said: "It is experiencing unprecedented demand with 2,600 hits per second, equivalent to 9.3 million hits per hour."
She said BT, which hosts the web service, was increasing capacity and it would have quadrupled by 5.30pm this afternoon.
"The technology for the call centres and the anti-viral collection points are working normally," she added.
A spokeswoman for the Patients Association said: "The Government and Department of Health has assured us that they are putting everything in place to make sure that patients and the public can have total confidence in being able to contact services at any time.
"However we have had evidence already that this is not so.
"While we accept that the new system may have had an extremely high number of people contacting it, it's disappointing to hear that the service was not able to meet demand."
www.direct.gov.uk/pandemicflu(National Pandemic Flu Service)
