Little effort is made to track down illegal immigrants who lose touch with the authorities, a senior immigration official has admitted.
The target for removing illegal immigrants has been met
David Roberts, of the Immigration and Nationality Directorate (IND), said there was little point hunting individuals who overstayed their visas.
He also said he did not have the "faintest idea" how many illegal immigrants there were in the UK.
The Tories said his words and those of other IND officials "beggared belief".
But Mr Roberts, who is head of removals at the directorate, said the directorate's resources were better targeted on firms employing illegal workers.
He told MPs he did not "have the faintest idea" how many illegal immigrants there were in the UK, although he was aware of estimates suggesting the number of illegal immigrants was about 400,000.
But he also admitted he did not know how many people had been told by the Home Office to leave the country.
"In terms of the number of letters sent to people who have been refused permission to stay here, I simply wasn't able to get that information," he told the Commons home affairs committee.
The situation was branded "amazing" and a "mockery of the immigration control system" by Labour MP David Winnick.
Mr Roberts said he could "understand the public's frustration" but the system would be tightened up when new "electronic borders" came into effect.
The IND's efforts were focused on removing failed asylum seekers and on mounting "targeted" and "high profile" raids on firms employing illegal immigrants.
"We are making huge efforts to remove them, but not on the basis of tracing individuals," Mr Roberts told the Home Affairs Select Committee.
He added: "There was a time, quite a few years ago, when I was an immigration officer, when indeed we knocked on lots of doors, following up lots of individual cases.
"And it came as no surprise that none of those individuals were at the addresses we had for them."
He said a "far more sophisticated, intelligence-led approach" was now used to find people who had overstayed their visas and were working illegally.
But unless they posed a "risk to national security," he could not accept pursuing individuals was an "effective strategy", he told the committee.
Committee chairman John Denham asked what a worried constituent ordered to leave the country should be advised to do.
"I get the impression the correct answer for me to give would be 'not very much, they don't track individuals. I wouldn't worry about it'," said Mr Denham.
Mr Roberts replied that the IND did a lot of operations targeted at where people work and kept in contact with people "as part of a reporting regime".
But he added: "If the committee concludes that we should be tracking individuals as part of its inquiry then that presents a series of challenges to us, to act on that recommendation in relation to the internal controls we have in the UK."
He said the IND had carried out more than 3,500 raids on companies last year.
And although this had only resulted in 293 successful prosecutions, it was expelling more than 1,000 illegal immigrants per month - which he said was well in excess of its target.
He said the UK's illegal immigration problem, which according to one estimate was about 0.7% of the population, was small compared to the United States, where they made up "7 or 8%" of the population.
But he admitted public confidence in the immigration system was fragile.
"Not having public confidence and allowing perceptions of a lack of enforcement I think is a huge challenge for us that we need to address," he told MPs.
Commenting on Mr Roberts' evidence and other revelations that National Insurance numbers were given to people without checking their immigration status, Shadow Home Secretary, David Davis, said: "This sounds like deliberate negligence on the part of the Home Office.
"There is no excuse for not tracking every one of these individuals until they are sure they are out of country.
"As for limiting their actions to those they consider a security risk, it is hard to believe that the Home Office is in a position to assess the threat to the public safety of every single one of 250 thousand failed asylum seekers currently still at large in this country.
"It beggars belief that along with our porous borders, problems with foreign criminals and our inability to deport a quarter of a million failed asylum seekers we also manage to hand out National Insurance numbers without checking up on a person's immigration status."