Mr Vine said the agency had failed to routinely or consistently make security checks to try to trace some of the unresolved cases that had been subsequently placed in the "controlled archive", a list of people UKBA officials believed they could not trace.

BBC: UKBA attacked over 'inaccurate' data given to MPs

"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, " he said.

BBC: UKBA attacked over 'inaccurate' data given to MPs

The MPs said the failure to trace these other migrants, whose cases were placed in a closed "controlled archive", led officials to conclude that they were not in the UK when in fact tens of thousands of them could be.

BBC: Border agency condemned over backlog