It could take ten years to remove all the illegal immigrants in the UK, Home Office minister Tony McNulty has admitted.
His comments came after Tony Blair said that nobody knew how many illegal immigrants were in Britain, warning any estimates were likely to be "highly speculative".
In the House of Commons yesterday, the prime minister said that "by its very nature, illegal immigration is very difficult to measure", but insisted that policies such as the electronic borders scheme and ID cards would help to address the problem.
Explaining the e-borders scheme last night, Mr McNulty said it would allow the electronic swiping of people in and out of the country, and gauge "far more readily the people coming in and out of the country" than the traditional paper system of paper visas.
"The Conservatives began to remove embarkation controls in 1996-7 and we completed the exercise.when you are faced with 90 million visits in and out of the country every year, that sort of paper based system is not appropriate," he told BBC Radio Four's PM.
Speaking later, he said the government was removing about 25,000 illegal immigrants and failed asylum seekers every year, but given the large numbers of people thought to be in Britain illegally, clearing the backlog would inevitably take time.
"We cannot have an exact figure [for people remaining] but I would say somewhere between 310,000 and 570,000," he told Newsnight.
"Assuming that we can find them and assuming that people are not going away of their own accord it would take some time to remove them all.we could be looking at around ten years to remove them all."
Mr McNulty said the best way to tackle the problem was to improve cooperation across Europe, which was being planned through the new EU asylum policy.
This would introduce common deportation procedures and spread the granting of refugee visas across the union.
"But most of the pressure is coming from overstayers and other types of illegal immigrants. It is at the work level where we can pick up many of these illegals," he added.
In the Commons yesterday, Conservative leader David Cameron said the government's failure to deal with illegal immigration after nine years in power was "unacceptable", and warned it was yet further proof of an administration "in paralysis".