The damning report follows comments by the chairman of the Home Affairs Select Committee, Keith Vaz, earlier this month.
Mr Vaz said the number of unresolved cases that UKBA is dealing with - more than 300,000 - is "spiralling out of control".
He said: "There are now about the same number of cases awaiting resolution by UKBA as there are people living in Iceland."
Chief inspector of the UK Border Agency John Vine has made the following recommendations to the UK Border Agency:
- Routine checks against police records.
- Ensuring that accurate information is given to the Home Affairs Select Committee.
- Making a public commitment to resolve the backlog within a certain timeframe
Chief inspector of the UK Border Agency John Vine was asked to evaluate how well UKBA had handled the backlog of thousands of unresolved immigration cases.
Mr Vine said he believed little had been done to try to resolve the cases before they were passed over.
He said: "Through the inefficiency and delay of the agency, those who would otherwise have faced removal will have accrued rights to remain in the UK."
Mr Vine also criticised "poor" customer service, and said that a lack of resources meant that deadlines were often missed, even when legal action was threatened.
The report said: "The issue of limited resources also created a significant impediment to case clearance.
"As a result, timescales given to applicants or their representatives about the resolution of cases were frequently missed, even where litigation was being threatened."
- UK Border Agency (UKBA) staff dealt with a backlog of immigration cases so inefficiently that at one point 100,000 pieces of post were unopened.
- In March 2011, there were 147,000 unfinished cases that were passed to an audit unit tasked with dealing with the backlog.
- More than 150 boxes of post, including letters from applicants, MPs and lawyers, lay unopened.
- In a sample of 135 files examined as part of the inspection, each case had lain dormant for an average of 87 months before they were reopened in 2010 for consideration.
- The shortest period of inactivity was six months and the longest period of inactivity was 17 years and nine months.
- A total of 115 cases were found to have entered the UK illegally, and there were only 10 cases where active efforts had been made to trace absconders.
UK Border Agency (UKBA) staff dealt with a backlog of immigration cases so inefficiently that at one point 100,000 pieces of post were unopened, a report said today.
Chief Inspector of Borders and Immigration John Vine found that security checks were not properly carried out on old cases and that workers had failed to check the records of other Government departments.
Applications were placed into an archive of unresolved cases after "very minimal work", despite the agency assuring MPs that "exhaustive" checks had been carried out, he concluded.
The number of outstanding immigration and asylum cases rose by 25,000 in three months and is "spiralling out of control", making the total caseload the equivalent of the population of Iceland, the Commons Home Affairs Select Committee Chairman Keith Vaz has said.
More than 300,000 cases needed to be dealt with at the end of June, up 9% over the previous three months, the Commons Home Affairs Select Committee revealed.
But clearing the backlog should not be a rushed job done without the proper checks, the committee's report on the UK Border Agency (UKBA) warned.
Ali was reportedly jailed for three years at Sheffield Crown Court and released in 2008, when the Home Office ordered that he return to Sudan and he was locked up in an immigration removal centre.
But he appealed to an immigration court and though a judge rejected his bid, he mounted a fresh appeal to the Upper Tribunal Immigration and Asylum Chamber, the Mail on Sunday reported.
He was allowed to stay because deporting him, the court ruling showed, would be contrary to the United Kingdom's obligations under the European Convention on Human Rights.
The UK Border Agency has reacted with fury to a court ruling allowing a Sudanese asylum seeker who raped a 12-year-old girl to remain in Britain.
Sani Adil Ali, 28, originally from Darfur and part of a threatened tribe, originally came to Britain in 2003 and was awarded refugee status in February 2005, it was reported.
But only a few months later he was arrested at his home in Middlesbrough and later admitted one count of raping the girl, who was Hungarian.
Prime Minister David Cameron has promised to take action on the legislation that helps foreign criminals stay in the UK.
Shadow Home Secretary Yvette Cooper said the Home Secretary needed to "to get a grip" this evening. She said: