By April 2008, people in 135 countries - three-quarters of the world's population - will need biometric visas before they can come to the UK.
Fingerprints are taken digitally and checked against a UK database
Only a very small number of individuals from these countries, such as heads of state, will be exempt.
The Home Office calls it "exporting the borders" - stopping people even before they leave their home countries.
The system relies on 2,300 UK visas staff working overseas to gather the biometric data.
This consists of digital finger scans - the 21st century equivalent of the old ink fingerprints - and a full-face digital photograph.
No more ink
In the past, visa applications could be made by post or through a travel agent without individuals ever physically coming to a visa section.
Now, once the basic forms are filled in, applicants in 53 countries are called to dedicated visa application centres (VACs) to submit their biometric information.
In other countries without VACs, would-be travellers must go to the British embassy or high commission instead.
Gone are the days of inky pads, today "prints" are taken by pressing fingers on a glass screen which looks like a small computer scanner. In fact, it takes a very detailed digital photograph.
The images of all 10 fingertips are then sent straight to London to be checked against the central government database and in five minutes the clearance officer in country can find out whether or not to grant the visa.
A Home Office spokeswoman explained: "It can tell you, 'Did this person claim asylum and was refused? Have they previously been caught working illegally or overstayed their visa? Are they a concern in any way?'"
The information becomes useful again to immigration enforcement officers carrying out operations in the UK to root out illegal workers.
"Staff take scan readers with them - they look a bit like games consoles. Then when they get to say, a factory or a restaurant, they can check people's fingerprints there and then," the spokeswoman explained.
"Someone might say, 'I've got a work visa', but in five minutes the officer will know if actually they've only got a tourist visa. Then we can detain them there and then."
By April 2008, 135 countries will be covered
2,300 UK visa staff worldwide
More than 500,000 sets of finger scans collected so far
Scans now collected at a rate of 100,000 a month
Biometric visas were first trialled in Sri Lanka in 2003 and then rolled out to east Africa in 2004. This was to help root out bogus asylum seekers claiming to be from Somalia, but actually originating from neighbouring countries like Djibouti and Ethiopia.
By July this year, prints were being collected at a rate of 100,000 a month from 93 countries.
And the government says they will help prevent identity theft, terrorism and organised crime.
The information is kept in accordance with strict data protection laws - it is not, for example, checked against the police national computer.
The Home Office also stresses that biometric visas are just one tool in its arsenal.
It has airline liaison officers based in overseas airports who check the documents of anyone boarding a flight to the UK. In the last five years, 180,000 people have been turned back at this point.