A website is inciting the murder of David Trimble, the former Ulster Unionist leader has claimed.
David Trimble urged the closure of the website
The site supports the 32-County Sovereignty Committee, believed to be the political wing of the Real IRA which carried out the Omagh bombing.
Lord Trimble has asked Metropolitan Police chief Sir Ian Blair to take action over the website.
He said the site contained "disgusting libellous statements" and "a clear incitement to murder me".
Last week, the US government told relatives of some of those who died in the 1998 Real IRA attack that it would do what it could to shut down the website.
Lord Trimble said it was "a considerable disappointment to see that the site continues to operate".
"I have no doubt that if this website was a Fundamental Islamist site advocating murder, arrests and closure would now have happened," he added.
Lord Trimble said it was a "grave mistake for the authorities to permit the continued operation of what is in effect a terrorist support group".
"Whatever complacency there might have been within mainland police forces about this sort of activity must surely have ended after last year's bombs in London and I do hope that vigorous action will now be taken on this matter."
He said he was copying the letter to the prime minister, home secretary, Northern Ireland secretary and Scottish secretary.
Meanwhile, the SDLP raised the issue of the site with US special envoy Mitchell Reiss during a meeting on Friday.
West Tyrone assembly member Eugene McMenamin said: "Mitchell Reiss confirmed that this matter was being looked at by the FBI and the American criminal authorities."
Last week, Michael Gallagher, whose son Aidan died in the Omagh bombing, said a notice board on the site was particularly offensive.
The website's service provider is in Canada, but it has a sister company in the US.
They asked Dean Pittman, the US consul general to Belfast, to try and get the Canadian government to move to shut down the site.
Lord Trimble has called for police in Scotland and Northern Ireland to become involved as there was a "clear connection" with Londonderry on the web page.
He added that he believed the server was operated through Glasgow.
Twenty-nine people, including a woman pregnant with twins, died in the 1998 Real IRA car bomb attack in the County Tyrone town.
Dr Kevin Curran of the School of Computing at the University of Ulster said it was difficult to get a website shut down "because they are not just within one jurisdiction".
"We have to deal with different countries and different laws," he said.
He said most internet service providers had acceptable use policies.
"But there is a worry that 'defamation havens' could spring up in jurisdictions which do not have defamation law," he added.
Dr Curran said websites with offensive material could theoretically be based in those countries, although he added that this so far had not happened.