LONDON: Immigrants should pay a bond of 5,000 pounds to cover costs of using public services before they are are issued visas to come to the UK, a key ally of Prime Minister David Cameron has suggested.Tory MP Nick Boles, a friend and former aide of Cameron, has urged the Government to impose a "surety" on migrants before granting them visas.This would be returned only if they paid several times more on tax than the value of their deposit, he said.Boles said the Government needed to go further in making sure immigrants contributed to society or risk spread social unrest. He said immigration had been "too high in recent years.""We need to make sure people who come here make a financial commitment to the country which they'll get back in a few years time. "One proposal is that they make a deposit that they forfeit if they commit a crime, if they're convicted of a crime or if they don't pay tax in the next three years."Boles made his radical suggestions in a book, 'Which Way's UP: the Future for Coalition Britain and How to Get There.'He called for an annual cap on non-EU immigrants of 20,000 to 50,000.The Government has imposed a cap of 24,100 but this applies only to skilled workers.Last year 190,640 foreign workers and their dependants moved to Britain, despite unemployment hitting 2.5 million.Boles suggested unemployed EU migrants should be expelled - an act he claimed would be legal under treaties.He also wanted language tests for all migrants and a restriction on access to social housing for at least five years.A spokesman for the Prime Minister said: "We are addressing concerns about immigration. We have imposed a cap."Our positions are not really that far apart. We are looking with interest at what he is proposing but we are not endorsing it."