Since the abolition of embarkation controls, which started in 1994, no Government have ever been able to produce an accurate figure for the number of people who are in the country illegally.
By the end of December 2008, the majority of foreign nationals will be counted in and out of the country.
This is one part of the biggest shake up of border security and the immigration system in a generation which also includes the global roll-out of fingerprint visas, compulsory watch-list checks for all travellers from high risk countries before they land in Britain and ID cards for foreign nationals.