Almost 9,000 migrants in the country unlawfully have been able to take advantage of a little known loophole that grants them permanent residence if they are not discovered for a set period of time.
It means they now have the same rights as a British citizen, including full access to the welfare state, despite being here illegally for almost a decade and a half.
They can also apply for a British passport.
The regulation was formally introduced by Labour in 2003 and critics last night said it had created an “amnesty in slow motion”.
A further 22,600 migrants have been granted indefinite leave to remain after being in the country lawfully for ten years even though they were not on visas that lead to settlement rights.
The figures, slipped out to MPs in a parliamentary written answer, come as the immigration service came under a fresh attack for its inability to efficiently tackle illegal migrants.
John Vine, the chief inspector of the UK Border Agency, said last week that the public report immigration abuses at a rate of 300 a day but officials were not even able to tell him how many calls resulted in action or an arrest.
He criticised the handling of intelligence as “unacceptable”.
In February last year, before the new Coalition Government took power, Ann Abraham, the Parliamentary Ombudsman, said UKBA could not perform even “basic” functions and is a “very long way” from effectively removing failed asylum seekers.
She said the agency risked losing public faith.
The rules allowing illegal immigrants to claim residence after 14 years previously existed as a loose concession but were formalised by the Home Office in 2003.
It means anyone who had managed to live undetected on the black economy for that period of time can then apply to the Home Office for “indefinite leave to remain”.
Only those with a bad criminal record effectively stand a chance of being rejected.
Since 2003, some 8,948 illegal immigrants have been granted a permanent stay and the rate is increasing.
Up to 2009 the annual approval rate ranged between 700 and 900 a year but increased to 2,061 in that year and 1,774 last year.
In 2005, a report for the Home Office estimated there could be up to 570,000 illegal immigrants in the UK although others have since suggested it could be as high as one million.
Migration Watch UK said numbers granted a stay under the rule could rise significantly as the effects of Labour’s relaxed immigration policies take effect.
Chairman Sir Andrew Green said: “The concession for illegal immigrants should be reviewed.
“It amounts to a slow motion amnesty. There is no reason why illegal residence in Britain, however long, should qualify someone to the full benefits of the Welfare State.”
Under the rule, illegal immigrants only have to wait four years longer than the period of time required of lawful immigrants to claim indefinite leave.
Those who stayed on a visa which does not lead to settlement rights, such as student visas, have to wait 10 years before applying for the same concession.
Since 2003, some 22, 635 lawful migrants have been granted a permanent stay under the rule.
Sir Andrew added: “These numbers will need careful watching.
“The government are now requiring students to show progression before they can extend their visas but there will be a backlog of “professional students” who will still be able to claim permanent settlement.”
A Home Office spokeswoman insisted illegal immigrants were not guaranteed indefinite leave under the system.
However, they only have to be of good character and speak English, which is highly likely after 14 years, to stand a very good chance.
The spokeswoman also hinted the system could now be reviewed.
“This Government has pledged to break the link between temporary migration and permanent settlement,” she said.
“Settling in Britain should be a privilege, not an automatic right.
“Completing 14 years continuous residence does not give applicants an automatic right to be granted Indefinite Leave to Remain.
“Each application is considered on its individual merits; applicants must demonstrate knowledge of language and life in the UK and be of good character, conduct and associations.”