Up to 250,000 people drop off the radar and end up as illegal immigrants in Britain every year, according to secret Home Office figures, amid warnings that border controls are completely ineffective and will only get worse after Brexit.

David Wood, who was head of immigration enforcement at the Home Office until 2015, said that more than 1.2million illegal immigrants are currently living in Britain, predominantly after overstaying their visas.

People are not aware of the scale of the problem because official figures are not published and Home Office estimates for illegal immigrants are kept secret by ministers because they are "embarrassing", according to experts.

In a highly critical paper for Civitas, written with Alasdair Palmer, a former Home Office speechwriter, Mr Wood disclosed that the Home Office has privately estimated that between 150,000 and 250,000 foreign nationals fail to return to their home country when they should.

They include people who come to the UK illegally in the back of lorries, those who are refused asylum (around 26,000 last year) and students and others who overstay visas.