At least 150,000 migrants have been refused permission to stay in the UK but the authorities do not know how many have left, a watchdog said today.
John Vine, chief inspector of the UK Border Agency (UKBA), said there is no clear strategy to find out what proportion of this growing number of migrants is still in the UK illegally.
Tracking these absconders down and removing them from the country is not seen as a priority for the agency, and there is no clear plan to ensure the cases do not just become another backlog to deal with, Mr Vine said.
At least 150,000 migrants have been refused permission to stay in the UK but the authorities do not know how many have left
He added: "There are over 150,000 cases nationally of migrants who have been refused an extension of stay in the UK.
"The agency does not know how many of these individuals have left the country or who are waiting to be removed.
"I also saw no evidence that there is a clear plan in place for the agency to deal with this stream of work to ensure this does not become another backlog.
"I believe it can and must do more to demonstrate it is dealing with this issue in a more proactive manner."
The extent of the problems emerged in an inspection of the Hampshire and Isle of Wight local immigration team at the end of last year.
Cases involving migrants in the UK, such as students, who had been refused an extension of stay were put in a so-called migration refusal pool and told they must leave within 28 days, the report showed.
Migrants in the pool who were still in the UK included those who should have left but had not done so, those who had applied for leave in another category, who had outstanding appeals or other legal barriers, or who had left the UK voluntarily by a route not captured by e-borders.
Staff underestimated the scale of the problem by about two-thirds, gauging there were between 400 and 600 cases when there were 1,893 in the area on December 12, Mr Vine said.
While the proportion of cases in the local team's pool remained at about 1.2% of all national cases between October and December, the number nationally increased every week from 153,821 on October 17 to 159,313 on December 12, UKBA figures showed.
An analysis of 44 cases by the inspectors found less than half had left the UK, 20 voluntarily and one as an enforced removal.
Of the remaining 23, the inspectors said the "greatest concern" was that the absconder tracing process was not always followed.
Nine of the absconders had not been added to the police national computer as either "wanted" or with a "locate trace" marker.
Some of the 23 were making further appeals to stay, were serving prison sentences or awaiting travel documents, the report showed.
Mr Vine also raised concerns that the effectiveness of the intelligence used to support arrest visits was not measured to ensure an efficient use of resources.
But he added that his inspection of the local team, which was the first such team to be established when it was formed in 2008, showed it was exceeding its removal targets and asylum decisions were made promptly in most cases.
Immigration Minister Damian Green said there had been no effective strategy under the last government to ensure migrants left at the end of their time in the UK.
The UK Border Agency was now working to identify those who had not left, he said.
He added: "This summer the UKBA launched a UK-wide operation to remove overstayers and we have already seen 1,800 removals since the campaign started.
"We are also working closely with other Government departments to create a hostile environment which makes it much harder for migrants to live in the UK illegally."
Mr Green is due to answer questions on immigration policy from the Commons Home Affairs Select Committee next Tuesday.
Committee chairman Keith Vaz said: "I am astonished that the UKBA has no idea where 159,000 individuals, the size of a city like Oxford, have gone since their application was rejected.
"The committee has not been given this information despite asking for the number of migrants who are untraceable."
He went on: "This is yet another group of cases we did not know about.
"We will be asking the Immigration Minister Damian Green on Tuesday if and when he knew about this new backlog, and what action the UKBA is now taking to find out if these individuals are still in the country.
"We do not want to see another six years spent finding foreign nationals in Britain.
"As the committee have said time and time again, the UKBA should spend more time locating illegal immigrants rather than targeting genuine migrants like students, spouses and family visitors."
Sir Andrew Green of Migration Watch UK said: “The ability to remove those who have no right to be in Britain is fundamental to the credibility of the whole immigration system"
Shadow home secretary Yvette Cooper said: "The damning conclusion from the Government's own immigration inspector has concluded that the Government is giving a very low priority to finding and removing people who have been refused permission to stay.
"Out of 150,000 people refused leave to remain, the Government seems not to know or care how many are still here."