Visitors to the United States will soon be fingerprinted and digitally photographed under a new hi-tech security system, a top US official has said.
The rules will not apply to UK citizens on holiday
The system will check the images against lists of known terrorists and those with criminal convictions or visa violations.
The new rules, which begin next January, will affect all travellers who need a visa to enter the US - about two-thirds of visitors.
Foreigners will for the first time be screened when they leave the country, as well as when they arrive.
"In 99.9% of the cases, the visitor will simply be wished a good day and sent on their way," said Asa Hutchinson, under secretary of the department of homeland security.
"But with that small percentage of hits, our country will be made much safer and our immigration system will be given a foundation of integrity that has been lacking for far too long."
He said at least two of the 19 hijackers in the 11 September 2001 attacks could have been stopped in this way. One held a student visa but did not attend classes.
Another, suspected ringleader Mohammed Atta, had previous visa violations.
The way you walk
The new rules will apply to those entering the US at an airport or seaport. It will be expanded to land border crossings in 2004.
Countries whose citizens do not need a visa will be urged to adopt "biometric identification" methods as soon as possible.
Those 27 countries - all part of the "visa-waiver" programme - include the UK, France, Germany, Australia and Japan.
The US is trying to develop ever-more sophisticated methods to thwart would-be terrorists, including facial recognition and iris scans.
The Pentagon is developing a radar-based surveillance system that can identify people by the way they walk.
Working on the theory that everyone's walk is as unique as a signature, the Pentagon has financed a research project at the Georgia Institute of Technology.
"There is a signature that's somewhat unique to the individual," the lead researcher, Gene Greneker, told the Associated Press news agency.
He said the technology could be used, for example, by security officers at an embassy to conclude that a shadowy figure loitering round the building three times in a week was the same person and should be investigated to see if he was planning an attack.
The Pentagon is calling its planned surveillance system Total Information Awareness, and it has already sparked alarm from civil liberties groups.
In February, Congress barred its use against American citizens without further congressional review.