The number of extremist websites espousing violent or racist views has grown by more than a quarter since January, according to a global study of "hate" sites.
The unprecedented 26% increase in the first four months of this year was almost as much as the growth in extremist sites during the whole of 2003, according to SurfControl, a British-based web filtering company.
Religious hatred appears to be flourishing on the web, with an increase in American religious sites advocating extremist views and a growing number of militant Islamist sites.
The number of violent and extremist sites monitored by the company's researchers in 15 countries, including the UK, has risen by almost 300% in the past four years. They detected just 2,756 hate sites in 2000, compared with 10,926 recorded in April 2004. Researchers also noted a convergence between sites advocating hate and those directly promoting violence.
Steve Purdham, the chief executive of SurfControl, said most of the extremist sites were created by American groups.
"There are two basic reasons for the growth. One is the relative growth of the internet. The other is the impact of headline news, in particular with regard to Afghanistan and now Iraq. In times of unease you tend to get extremist views forming and the internet is a global forum for this."
Offensive sites that have flourished in recent months include those bearing anti-American messages, as well as sites promoting the idea of Jewish conspiracies, pictures of mutilated people, revisionist versions of the September 11 terrorist attacks and dating and scholarship services for white supremacists.
Other news events that appear to have triggered the recent sharp increase in hate sites include the controversy over gay marriages and the release of Mel Gibson's The Passion of the Christ, which has been used by some extremists as a platform to express hatred of non-Christians.
US extremists, including anti-abortionists, have published home addresses and phone numbers of officials and doctors on the internet with thinly veiled incitements to violence.
In Britain, an animal rights website was recently removed from the internet after activists posted the names, home addresses and phone numbers of more than 100 famous people deemed to be pro-hunt under the headline "Celebrity Bloodsports Scum".
A spokesperson for the Internet Watch Foundation, the UK's monitoring body, said it had seen a 101% increase on the previous year in complaints of racial abuse made through its hotline.
"We would fully support any initiatives from the government on how to deal with such content more effectively," the spokesperson said. "We would also like to see some successful international consensus on how to deal with illegal content which is hosted overseas, but available to internet users in the UK."
If a site is deemed criminally racist under UK law and hosted by a UK-based internet service provider, the IWF can order the service provider to close the site. If the service provider does not close the site, they can be prosecuted for "publishing" it. Websites can also be prosecuted under public order legislation if they are deemed to stir up racial hatred against a group of people in Britain.
But UK regulators have no power to remove hate sites hosted in other countries. In 2003, 55% of all illegal images detected by the IWF came from America, with 23% from Russia, a growth area for violent internet pornography.
John Carr, the internet adviser to children's charity NCH, recommended that people buy filtering software for their computers to minimise their contact with such material.
"You can't make the internet subject to the same sort of censorship as in British cinema and you can't make net publishing subject to the same kind of controls as printed publications," he said.
"In the absence of that, you need mechanisms to protect the young and the innocent from exposure to this complete garbage."
Mr Carr called on the government to insist that companies sell home computers with filtering software already fitted.
As well as using filters, Mr Carr said, young people should be taught to look critically at information on the internet and not take sources at face value.