Border officials failed to pursue more than 3,000 leads identified on police databases when attempting to track down missing asylum seekers, an inspector has found.
The now-defunct UK Border Agency (UKBA) took no action to locate some 3,077 positive hits returned when matching asylum seeker details on the Police National Computer (PNC), Independent Chief Inspector of Borders and Immigration John Vine said.
Border staff told the inspector that a decision was taken not to write to applicants in relation to PNC checks because the information was deemed unreliable.
The inspector added that he was "not provided with any rationale to support this view".
And, had the PNC checks been followed up, it might have resulted in new information coming to light that would have helped the UKBA to locate individuals.
Mr Vine was asked by the Home Secretary to conduct a follow-up investigation after a November 2012 report found a number of serious failings at the UKBA.
The chief inspector discovered a number of improvements but still had some concerns.
As well as the failure to follow up PNC checks, Mr Vine said there were some cases where information contained in paper files was not being used to trace applicants.
And work had not yet commenced on archived cases and active reviews that had been reopened as a result or positive data matching results.
Mr Vine said: "There were still a number of cases where there was information contained in paper files which could have been used to trace applicants.
"I believe the Home Office needs to demonstrate to applicants, Parliament and the public that it has taken all reasonable action to identify whether individuals remain in the UK illegally.
"While action had been taken to reopen archived cases following positive data matching results, I was concerned that no work had actually started on them. This was also true of active reviews.
"The Home Office will now need to ensure that these cases are afforded priority and publish a realistic and achievable timescale for the completion of all legacy asylum and migration cases."
The UKBA was abolished earlier this year after a raft of damning reports, including some based on inspections conducted by Mr Vine.
The Agency was split into two - the UK Visas and Immigration Section and an immigration law enforcement organisation - and brought back under ministers' control.
Earlier this year, the Home Affairs Select Committee warned it would take the UKBA 24 years to clear a backlog of asylum and immigration cases the size of Iceland's population.