The government has established a target of reducing UK immigration figures to the "tens of thousands" before the next election. It intends to do this by limiting the number of UK work permits on offer and by imposing stricter regulations on spouse visas and student visas.
However, at the moment the government simply doesn't have the accurate data it needs to keep tabs of how many people are entering and leaving the country. The e-borders programme, launched in 2003, was supposed to solve this problem by collecting details from passenger lists of everybody entering and leaving the country.
Unfortunately, the system is still not up and running accurately and although all flights from outside of the EU are now covered, ports and railways are not expected to be included in the e-borders system until 2014, while EU flights are unlikely to be covered until 2015 at the earliest.
Until the system is up and running, the majority of the current figures are based around the International Passenger Survey, which was designed in the early 1960s. It operates by selecting every 30th or 40th passenger coming through arrivals or departures and asking them if they wouldn't mind being surveyed. This raw data is added to information regarding asylum seekers, migration statistics from Northern Ireland and the number of people in the country on short-term visas in order to calculate a very rough estimate.
Officials are critical of the a succession of government failures to improve the system. Dr Martin Ruhs, director of the Migration Observatory at the University of Oxford, told the BBC: "It is very difficult to assess how well the government is progressing toward its target of reducing net-migration to the 'tens of thousands', or to evaluate the effects of specific policy changes.
"In simple terms the government could miss the 'tens of thousands' target by many tens of thousands and still appear to have hit it - conversely the government could hit, or even exceed its target and still appear to have missed it."
It will take time to improve the system, but an accurate method of measuring UK immigration is urgently required if the government is to have any chance of gaining control of the system.