There are nearly three quarters of a million illegal immigrants in Britain, research has suggested.

Telegraph
"The last official estimate of illegal immigration, a Home Office report in 2001, put the figure at 430,000. ...Because of the nature of illegal immigration, accurately charting numbers is difficult. The LSE team said the figure lies somewhere between 524,000 and 947,000, with a "midpoint" figure of 725,000.
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Number of illegal migrants in Britain is 80 per cent more than official estimates
"BBC's Panorama programme last night claimed that at current rates of deportation, it would take 34 years and cost £8 billion to clear the backlog of people who are currently in the UK illegally."