Some 6,000 potential immigration cheats have been caught by a scheme to fingerprint visa applicants before they travel to the UK.
The government wants prints from every applicant by April 2008
UKvisas said more than 500,000 sets of prints had been taken since the scheme was introduced last year, with 6,000 matched to "people of concern".
Compulsory fingerprinting forms part of the UK visa application process in just under 50% of countries.
The government hopes to collect prints from every applicant by April 2008.
Immigration Minister Liam Byrne said fingerprint visas were becoming the UK's "first line of defence against illegal immigration".
"By collecting fingerprint information on foreign nationals coming to the UK in advance of their journey, we are building a new overseas border control which is flushing out the false visa applicants," he said.
One applicant was prosecuted for deception because, in an earlier unsuccessful visa application made in Amsterdam, he had used a different name.
And a Sudanese businessman who said he wanted to travel to the UK for a shopping holiday with his wife was also caught out.
The application was refused after a check of his fingerprints revealed a failed asylum claim in 2004 under a different identity.
Foreign Office Minister Kim Howells said: "Recording biometric information allows us to fix an individual's identity at the earliest point practicable and track their future dealings with the UK."
Meanwhile, the Conservatives have accused the government of ignoring expert advice that its controversial identity cards scheme could be scuppered by technology problems.
Last month, Cambridge University IT expert Professor John Daugman warned that using fingerprints to identify individuals meant one in a thousand card-holders would be wrongly matched with other people's details.
He said: "I don't want to be very pessimistic and say this whole system is doomed to fail but I will say that just from analysing the mathematical requirements, if it is just fingerprint, it is unlikely to be able to succeed."
The Conservatives say that, by the time the entire UK population is on the ID cards database, there could be 40,000 "false matches".
Shadow home secretary David Davis said: "Gordon Brown pretends ID cards will secure our identities, but tens of thousands of people could be falsely accused of not being who they are.
"That will create, not solve, problems and divert time, money and effort from vital areas of public protection, including at our borders and in the fight against terrorism."
But a Home Office spokesman said it was "not true" to suggest the database could not cope with vast records.
In the US, the FBI had a system with 47 million records and the US-Visit scheme - which records fingerprints of those arriving in the country - held details from more than 80 million people, he added.
The spokesman added: "The National Identity Scheme biometric checking processes are expected to utilise two biometrics (face and fingerprint), as well as manual inspection and biographic checks where necessary.
"We have not ruled out the use of iris technology for the NIS. However, we are sensibly starting with fingerprints, as these are in any case being introduced in passports and immigration documents."