British Prime Minister Gordon Brown wants Internet companies to help stifle online terrorist propaganda, he told lawmakers Wednesday, as officials say they plan to meet leading service providers to find ways of putting a lid on extremist content.

But the providers argue they already do all they can to fight illegal terrorist material online, and experts say even powerful filters cannot block determined users from getting their message out.

"Fundamentally, it's a losing proposition," said Ian Brown, a research fellow at the Oxford Internet Institute, noting that even countries such as China and Myanmar have had trouble with their online censorship efforts.

• Click here to visit FOXNews.com's Cybersecurity Center.

• Click here for FOXNews.com's Personal Technology Center.

The prime minister's proposal comes as the European Union considers ways to sanction Web sites that display terror propaganda or recruit for terrorist groups.

Addressing lawmakers, the prime minister said Home Secretary Jacqui Smith was "inviting the largest global technology and Internet companies to work together to ensure that our best technical expertise is galvanized to counter online incitement to hatred."

The Home Office said it would meet leading British Internet service providers to examine ways of curbing online propaganda, but said Brown's plan had not yet been considered in detail.

Not clear, for instance, was whether the plan would require new laws or different ways of enforcing existing regulations.

British law already forbids the publication of statements likely to be seen as encouraging terrorism or the dissemination of terrorist material, such as bomb-making information, according to the Internet Watch Foundation, an EU-funded body that works with the British government to monitor and remove illegal online content.

Under so-called "notice and take down" procedures, authorities, companies and individuals can demand that Internet service providers remove content considered to be unlawful. That includes child pornography, as well libelous, obscene or terrorist material, the group said.

Although the removal of child pornography is relatively uncontroversial, service providers have expressed unhappiness at having to shut down their customers' sites over, for example, allegations of libel, where guilt is difficult to determine at a glance.

They are unlikely to welcome similar demands over material that allegedly glorifies terrorism.

Besides taking down their own customers' sites, service providers also might be pressured to block ones hosted abroad.

The government might draw up a list of banned sites, similar to one the Internet Watch Foundation has maintained since 2004 and updates twice daily to block Britons from visiting child pornography sites hosted overseas.

Another method might be to persuade search engines like Google Inc. or Yahoo Inc. to filter out prohibited content from their search results, or manage their searches so that the words "bomb," "al-Qaida," or "video" did not lead users to terrorist-related sites.

But both these measures would do little to deter the computer-literate youth being targeted by al-Qaida, Ian Brown said.

He noted that users could still swap terror-related content through file-sharing networks, discussion forums, or access material through sophisticated proxy servers and programs that allow users to browse the Net anonymously.

Efforts to use Internet service providers to police online content amounted to a "censorship proposal" and was bound to be problematic, said John Gage, vice president and chief researcher for Sun Microsystems Inc.

"It's one of these things that's going to be very difficult to implement," he said.