Scores of immigrants who were fraudulently issued with visas by a rogue UK Border Agency official remain at large almost four years later, a court has heard.
Nigerian Samuel Shoyeju, an immigration officer, was today jailed for seven years for falsely granting indefinite leave to remain to 44 non-EU residents who were not entitled to stay in the country.
Only 14 of the false letters that he issued have been recovered, meaning most of the recipients have not been identified.
Judge Christopher Mitchell said the case threatened to undermine the hard-won reputation for "probity and honesty" of British civil servants.
He added: "Actions like yours call into question in the public mind the entire integrity of the immigration system when, at the present time, immigration and immigration decisions are extremely sensitive."
Basildon Crown Court heard the "treasured status" allowed immigrants to live, work and claim benefits as well as applying for full British citizenship.
The recipients, who are all believed to be Nigerian, ranged from asylum seekers to those who had already identified as "overstayers".
Prosecutor Lucy Kennedy said: "Not all of those people have been traced."
She added: "This case has cost the taxpayer a significant amount of money, both in terms of the investigation and in terms of those who remain in the country untraced.
"He abused the system he was employed to protect and he did so for his own selfish reasons, not out of some misguided sense of altruism.
"It was a breach of trust which will no doubt undermine confidence in public servants employed to protect UK borders."
Shoyeju, who worked as a line manager in Croydon, amended and destroyed electronic and paper records to cover his tracks.
He used colleagues' date stamps and accessed secure databases under other people's names to divert attention away from himself.
When his home computer was searched a file called "Nigerian names" was found, relating to those he planned to grant visas to.
He was able to use a generic computer log-in to transfer secure files from Home Office systems without being identified, Ms Kennedy said.
"That system relies on the integrity of immigration officials themselves," Ms Kennedy added.
"He had been employed to uphold immigration rules but, using his intricate inside knowledge of the system, he deliberately did the opposite."
The 53-year-old, of Namur Road, Canvey Island, Essex, admitted misconduct in public office by falsely granting indefinite leave to remain between 2006 and 2007.
Although the prosecution has not been able to prove a financial motive, the court heard thousands of pounds were paid into his bank account during the time of the offence.
Stephen Linehan QC, in mitigation, said that there was not sufficient evidence that Shoyeju had a financial motive and could not be sentenced on that basis.
Judge Christopher Mitchell said: "An immense amount of public time and money has been spent investigating and preparing this case."
Shoyeju arrived in the UK in 1988 and was allowed to remain until 1991 as a student.
The married father-of-three was then granted indefinite leave to remain as the spouse of a British citizen and was granted full citizenship in 1996.
He began working as an immigration officer in 2002.