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The Home Office has been hit by a fresh controversy after a report today revealed 223,600 illegal immigrants had fallen off its radar.

Files on the missing migrants sat for years in unopened boxes in a Sheffield office.

Chief inspector of borders and immigration John Vine revealed the extra 223,600 cases had been quietly added to official figures when he launched an inquiry earlier this year.

And an urgent review found just 46,000 of them have since left Britain.

It means the total number of migrants who could still be living in the UK after overstaying visas has doubled to nearly 350,000.

Shadow Home Secretary Yvette Cooper raged: “Theresa May promised to tackle overstaying, but she has completely failed.”

Mr Vine’s report also revealed a £30million Home Office drive to remove the overstayers has failed.

Less than 1% of people targeted by private contractor Capita have left Britain as a result.

Mr Vine said: “Far fewer migrants have been persuaded to depart than anticipated. Of 120,000 people whose cases were sent to Capita, I found less than 1% had left as a result of Capita’s intervention.”

His review found the firm was hampered by the Home Office’s “highly inaccurate” data on the missing migrants, and in many cases the contractor simply sent out letters to the people concerned.

Mr Vine added there was “no evidence” Capita’s work had “increased the number of enforced removals” the Home Office had achieved.

The report is the latest in a series of scandals to rock Mrs May’s department, after the revelation foreign criminals have been granted British passports without proper checks.

Mrs Cooper said: “This report reveals yet one more border control failure from Theresa May to add to the catalogue of chaos at the Home Office.”

However, Tory Immigration Minister James Brokenshire insisted the situation has improved since 2010, adding: “We inherited an immigration system in complete disarray.”