The City watchdog has uncovered the largest known "suckers' list" of potential British victims which was being circulated by boiler-room fraudsters.
The Financial Services Authority (FSA) recovered the "master list" with names, addresses and telephone numbers of 49,387 people in the UK as part of an ongoing investigation in tandem with its US counterparts and the Inland Revenue.
The regulator is writing to everyone on the list – these are often dubbed "suckers' lists" – to warn them, and advise on how to avoid being scammed. So far it has contacted 95,000 people to inform them they are on a list.
Margaret Cole, the managing director of enforcement and financial crime at the FSA, said: "This latest list is the biggest we've ever recovered and we are contacting every single person on it in the hope we can stop people losing money."
Many of the intended victims are based in London, the regulator said, adding that there were also a significant number based in Scotland and the South-east of England.
Boiler-room fraud in Britain is estimated to cost £200m a year. The practice was the focus of the acclaimed film Boiler Room in 2000. Scammers contact victims and con them into buying shares that turn out to be worthless or even non-existent. It is rare for victims to recoup their losses.
The fraudsters are predominantly based abroad, with the regulator pointing to Spain and the US as particular hotbeds of the criminals, who are often linked to organised crime.
A spokesman for the FSA said it carries out raids, froze assets and chased the money around the world "but the nature of it makes it very hard to catch the fraudsters". The regulator estimates that the average boiler-room victim loses £20,000, based on the 1,000 victims who contacted it this year.
"It is estimated that only 10 per cent of victims report the crime," it added. The FSA has recovered three other suckers lists this year.