A deadline for states with visa waiver agreements with the US to start issuing a new generation of biometric passports to meet US demands has passed.
The UK is among 27 countries which have changed passports
Under legislation introduced after the 11 September attacks, passports issued in Visa Waiver Program countries after 26 October must contain biometric data.
Face-mapping technology is used to create the image stored in a chip on the passport in a bid to fight fraud.
Some civil liberties campaigners are concerned about the use of biometrics.
About 13 million people travel each year to the US under the Visa Waiver Program (VWP) - which entitles them to apply for entry to the US on a passport for up to 90 days for business or leisure without needing a visa.
If any of the 27 VWP countries issues a passport without the necessary criteria after 26 October, the holder will have to apply for a visa.
There will be no immediate impact on those travelling on Thursday, as the US will still accept passports issued before that date which have a digital photo and are machine-readable.
Washington initially set a deadline of 26 October 2005 but extended it by a year after the European Union complained many nations would have trouble completing the changeover in time.
The decision to use face technology, as opposed to fingerprint or the more accurate iris systems, was set out by the International Civil Aviation Organization in 2003 - as it was the most developed and widely available of the biometric systems at the time.
The UK Identity and Passport Service has issued more than 2.5m biometric passports since March and claims they are "the most secure passport ever issued by the UK".
Structural dimensions of facial features are taken from an applicant's passport photograph and converted into digital data which is stored on a chip in the document.
The chip also contains the biographical details found on the back page of existing passports. It is hoped they will be more difficult to forge.
David Starkie, of technology firm 3M which provides the passport issuance system, says there is a lot of suspicion around the use of biometrics.
US VWP PASSPORT DEMANDS
Issued after 26 Oct 06:
Biometric data and chip
26 Oct 05 - 26 Oct 06:
Digital photo (printed not glued)
Before 26 Oct 05:
None of above:
Apply for visa or new passport
"But if you do clone or copy this, what are you going to do with it, if it is not your face?" he said.
Automated iris recognition systems are being trialled in some countries, enabling pre-registered passengers to avoid passport control, but those travelling to the US will still face a security guard at the port of entry.
Travellers will present their passport to the official, who will swipe it, bringing up the details on a computer. The official can then check that the face scan on the chip matches the photograph on the passport, and the holder.
Statewatch director Tony Bunyan says that means the passports are not truly "biometric".
VISA WAIVER PROGRAM MEMBERS
Andorra, Austria, Australia, Belgium, Brunei, Denmark, Finland, France, Germany, Iceland, Ireland, Italy, Japan, Liechtenstein, Luxembourg, Monaco, Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovenia, Spain, Sweden, Switzerland, and UK - including Channel Islands and Isle of Man
"That is only good for a one-on-one check to confirm the person is who he says he is - but that is no good for a database."
He says the excuse that it will help prevent international terrorism is questionable as you need to know whose face or fingerprints you are looking for in the first place.
He also questions the plans for the introduction of other biometrics - fingerprints and iris scans, which will mean passport applicants having to present themselves at enrolment centres in person.
The EU wants biometric passports to include fingerprints by 2009 - which mean the rolling out of a second generation of biometric passports.
Dr Fred Preston, Director UK Identification and Security for Motorola, says he believes biometric systems will be used more and more to help secure borders.
"Multiple biometrics is the way to go, but following ICAO recommendations for face technology is a great way to start," he said.