Thousands of immigrants were allowed to stay in the UK without undergoing proper checks because of a mountainous backlog of cases, a report said today.
UK Border Agency (UKBA) staff dealt with immigration cases so inefficiently that at one point 100,000 pieces of post were unopened.
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.
Mr Vine said: "An examination of controlled archive cases showed that the security checks - which the agency stated were being done on these cases - had not been undertaken routinely or consistently since April 2011.
"I also found that no thorough comparison of data from controlled archive cases was undertaken with other Government departments or financial institutions in order to trace applicants until April 2012.
"This was unacceptable and at odds with the assurances given to the Home Affairs Select Committee that 124,000 cases were only archived after 'exhaustive checks' to trace the applicant had been made."
He was asked to evaluate how well UKBA had handled the backlog of thousands of unresolved immigration cases.
In March 2011, there were 147,000 unfinished cases that were passed to an audit unit tasked with dealing with the backlog.
Mr Vine said he believed little had been done to try to resolve the cases before they were passed over.
"I found that updates given by the agency to Parliament in the summer of 2011, stating that the legacy of unresolved asylum cases was resolved, were inaccurate," he said.
"In fact, the programme of legacy work is far from resolved. On the evidence I found, it is hard not to reach the conclusion that cases were placed in the archive after only very minimal work in order to fulfil the pledge to conclude
this work by the summer of 2011."
He said that at one point more than 150 boxes of post, including letters from applicants, MPs and lawyers, lay unopened.
Some asylum seekers who had no grounds to stay in the UK accrued the right to remain in the country because they were waiting so long for their cases to be resolved, the Chief Inspector concluded.
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."
In a sample of 135 files examined as part of the inspection, each case had lain dormant for an average of 87 months - more than seven years - 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.
Only 34 applicants had been recorded as absconders on the police national computer.
The Chief Inspector made a series of recommendations to UKBA, including routine checks against police records, ensuring that accurate information is given to the Home Affairs Select Committee and making a public commitment to resolve the backlog within a certain timeframe.
His damning report follows comments by the chairman of the Home Affairs Select Committee, Keith Vaz, earlier this month, who 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."
A Home Office spokesman said: "The backlog of asylum claims that was allowed to build up before 2006 was unacceptable.
"Since then, UKBA has worked its way through the backlog. Cases where, after checks, the agency was unable to trace the individual were placed in the controlled archive. Before they were put into the controlled archive, all cases were checked against the warnings index and the police national computer.
"UKBA's policy was that all cases in the controlled archive should be checked regularly against the warnings index and the police national computer. This report shows that these checks were not carried out.
"The UK Border Agency has now reviewed the cases within the controlled archive and undertaken a robust process to trace those we can find evidence are still in the UK and, where appropriate, remove them from the country."
The spokesman added that the agency was implementing "every recommendation from the report".
He said: "We have known for some time that UKBA is a troubled organisation with a poor record of delivery. Turning the agency around will take time, but we are making progress. The Border Force is now an independent organisation and its performance is improving. And UKBA has a transformation plan that will put the agency on a surer footing.
"A new performance and compliance unit has been created to provide assurance that information and data that is routinely published by the agency is robust and reliable. The Home Secretary will invite John Vine as part of his future inspection plan to inspect the audit and assurance mechanisms of the agency."
For Labour, shadow immigration minister Chris Bryant described the chief inspector's report as "utterly damning".
"The Home Secretary must immediately explain why her department's UKBA has provided incorrect information to the Home Affairs Select Committee and to Parliament as well as ignored requests for information from the select committee," he said.
"She must also explain why her staff have been so slapdash in their attempts to track down failed asylum seekers. She cannot hide behind others. This has happened on her watch."
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