Selling passports. It may sound illicit but 'citizenship-by-investment' is a global industry worth billions - and it's completely legal.

The idea is simple - invest huge sums of money and in return acquire residency rights or citizenship, even visa-free access to all European member states.

The UK offers residency in exchange for an investment of £2 million - or for £10 million, the possibility of British citizenship within two years.

And across the world, countries are vying to attract the super-rich through these schemes. But they are attracting attention for the wrong reasons.

European MEPs have launched an investigation into a 'Golden Passports' programmes across Europe - including the UK - amid concerns that they pose a corruption risk. In the US, government financial investigators say individuals are buying citizenship to hide their true identity, in an attempt to flout economic sanctions against Iran.

Tonight, File on 4 examines the trade in passports and visas for the wealthy and asks whether they deliver any real value for the countries that sell them, and assesses the evidence that they are being used by criminals.

Reporter: Alys Harte