The Civil Liberties Union reached the number by citing the 700,000 records on the list last September and adding 20,000 names each month, as forecast by the inspector-general of the Justice Department. The result is that if a terrorist uses the alias Gary Smith, for example, all Gary Smiths could be subject to extra scrutiny, such as searches and questioning at airports and border crossings.

Jim Robinson, who was an assistant attorney-general during the Clinton presidency, said he could not print an electronic ticket at home because his name matched one on the list, even though he still had a top security clearance. He said the Government's terrorist watch list had probably caused thousands of innocent Americans to be questioned, searched or otherwise hassled. "It's a pain in the neck and significantly interferes with my travel arrangements," he said. He believes his name matches that of someone who was put on the list in early 2005, and he is routinely delayed while flying - despite having his security clearances renewed last year.

"If I were convinced that America is a safer place because I get hassled at the airport, I might put up with it," he said. "But I doubt it." For some Americans whose names match those on the list, being delayed or detained for extra screening is not just a hassle - it is frightening.

A computer consultant from the Chicago area, Akif Rahman, said he had been detained at least seven times after travelling abroad. He said that in May he was shackled to a chair and interrogated for more than five hours at the US-Canadian border while his wife and two children waited with no access to food or a telephone. Mr Rahman is one of 10 people who have sued the Government, seeking to remedy policies that caused such detentions because they were misidentified. The Government calls its watch list one of the most effective tools in its fight against terrorism. It was created after the attacks of September 11, 2001, to consolidate 12 existing lists and prevent terrorists going undetected if they were entering the country or otherwise stopped for questioning.

Last year congressional investigators found "general agreement that the watch list has helped to combat terrorism". Other audits of the list have concluded that it has mistakenly allowed innocent people to be stopped because their names are similar to those on it.

As of October 2006 more than 30,000 airline passengers had asked the Homeland Security Department to clear their names from the list. Up to 20 suspected terrorists were left off the list as of last year due to a technology glitch. The list is compiled by the FBI and given to airlines, the Transportation Security Administration and US Border Patrol. Chad Kolton, a spokesman for the FBI's terrorist screening centre, which administers the list, said it was not available to the public so there was no way of knowing if someone was on it.

The Civil Liberties Union called on the President, George Bush, to issue an executive order requiring the list be reviewed and limited to those for whom there was credible evidence of terrorist activity. It also called for the right to see the list and challenge the data. Cox News Service, Associated Press