Users of TV set-up boxes that let viewers stream content expect a knock on the door could face years in jail for digital piracy.

Around 25% of Britons are thought to be accessing digital material illegally, but are totally unaware their TV set-up boxes are illegal.

More than one million of these have been sold in the UK in the past two years alone.

They have let young children see inappropriate advertising, exposed consumers to dangerous and counterfeit electrical goods and led to a rise in financial cyber crime, investigators say.

International organised criminal gangs behind the sales are profiting by millions of pounds, often from fraud and other serious offences.

Gangs typically use the dark web – hidden from the mainstream internet – to sell illicit information, such as the illegal software used to modify set-top boxes, or the customer data they have acquired through malware.

A damning report highlights the rise in streaming through Twitter and Facebook, which makes it easier for the criminals.

They make their money from advertising – typically banner ads or pop-up windows for casinos or dating sites, sometimes exposing children to harmful content. Other money-making scams include subscription fees to access paid-for channels and charging other criminals to put malware on sites and hijack users’ computers.

Kieron Sharp, director general at intellectual property protection organisation Fact, said: ‘This report has come at a crucial time in our fight against digital piracy.

‘A quarter of Brits access digital material illegally, and often don’t realise the risks associated with that, for them and their families. Pirates are not Robin Hood characters, they are criminals who do it to make money through illicit means.

‘As a result, the risks are high – inappropriate advertising that could be seen by young children, electrical safety associated with counterfeit parts, and financial cyber crime.’

Detective Chief Inspector Pete Ratcliffe, of the Police Intellectual Property Crime Unit, said: ‘While it may be tempting for people to think they are getting a bargain when streaming illegally, it’s important to remember that there are organised criminals behind it, often associated with other serious crimes.

‘Pirating content is not a petty crime; from release groups, to site operators to set-top box wholesalers and distributors, there is an international criminal business model.’