Home Office 'failed to deal' with foreign nationals refused leave to stay in UK
The government failed to deal with a backlog of almost 174,000 foreign nationals who should have been removed from the UK, a report has said.
The independent inspector of immigration said the Home Office must make significant improvements regarding "overstayers" - foreign nationals refused leave to stay in the country.
John Vine said failure to do so could "undermine public confidence".
Immigration Minister James Brokenshire said there was room for improvement.
The report suggests that the amount of "overstayers" in the UK is not increasing, and instead is remaining largely static.
Mr Vine said the overall size of the "migration refusal pool" - the number of foreign nationals refused leave to stay in the UK after 2008 - was 173,562 in the three months to June this year, compared with 174,057 in the same period two years earlier.
The Home Office signed a contract with private outsourcing group Capita to review, and where possible, close the records of migrants who had been refused leave to stay in the UK.
But Mr Vine said there had been "significant inaccuracies" in Capita's classification of migration refusal records.
The inspection concluded that of a sample of 57 records 16 had wrongly recorded the foreign national as having left the UK.
A Capita spokeswoman rejected claims that it had used information about passengers to the UK incorrectly and that it had incorrectly classified passport records.
She also rejected "any inference that we benefited in any way" from following the processes used to record migrants' entry to and exit of the UK.
In his report, Mr Vine specified that what he describes as Capita's inaccuracies meant the number of "overstayers" thought to have departed may have been overestimated by as many as 1,140 in 2013-14.
Mr Vine said: "I was disappointed to find a high level of inaccuracy in the classification of migration refusal records, with more than a quarter of departures in my sample being incorrectly recorded.
"Considerable improvements in the Home Office's capability to monitor, progress, and prioritise the immigration enforcement caseload will be needed to deliver its strategy for reducing the level of irregular migration."
He said inspectors had also identified a further 223,000 records of foreign nationals without permission to stay in the UK, whose cases dated to before 2008.
James Brokenshire said the report's recommendations would be taken forward, but changes made by the government had been having a positive impact and needed to be judged over time.
"The public can have confidence on the rigour that we are attaching to this issue," he said.