British immigration officials have admitted they have no idea of the whereabouts of 50,000 illegal immigrants, effectively granting them a “de facto amnesty”.

Immigration enforcement officials are also not even looking for most of them due to limited resources, leading to the Home Office having to hire a private firm, Capita, to track them down.

The revelations came when Mandie Campbell, director general of Immigration and Enforcement for the Home Office, gave evidence to a committee of MPs yesterday. She said that 121,000 immigrants were successfully tracked down, however another 50,000 slipped through the net. Those they have tracked down are now also lodging new cases to remain in Britain.

The Telegraph reports that she told MPs: “Capita do a range of checks on the individuals to follow up last known addresses and specialist checks with credit agencies. A range of checks take place to try to determine their location. That group we have had no contact with at all.

“We are not sending enforcement officers to that address, because we a limited amount of enforcement resource. We apply that resource on a harm basis. If there is any evidence that they are causing harms, ie criminal offending, then of course we would.”

Conservative MP Stewart Jackson replied: “You don’t know who they are, where they are, what they are doing? Do you think they are going to turn up at Portsmouth and say can I check in on your embarkation system please and leave the country?

“It’s just not going to happen. It’s not just 500 people, it’s 500000 who you have just lost. This is a serious public safety issue and there seems to be an element of complacency here – 50,000 might turn up, they might not. It’s not acceptable.”

The news comes as the government tries to put new anti-terrorism measures in place. On Monday, Prime Minister David Cameron announced a series of measures to combat the growing threat of British citizens fighting for ISIS. Under the plans, it will be easier for the government and border officials to confiscate the passports of citizens who travel to fight with the terror group, preventing them from returning home.

The inability of immigration officials to track down illegal immigrants, however, will lead to fears that extremists could slip through the net when they try to enter Britain.

Last night, dozens of migrants in the French town of Calais tried to storm a ferry heading for Britain. Their attempt was unsuccessful, but only after they were held back by staff using fire hoses.