The Financial Services Authority (FSA) has contacted more than 49,000 people to warn them that they are on a list used by fraudsters.
The biggest ever 'master list' used by boiler room fraudsters has been recovered, containing the names, addresses and telephone numbers of some 49,387 people who the FSA believes may have been contacted out of the blue and offered worthless shares.
The greatest concentration of targets is in London, although there are a significant number based in Scotland and the South East of England.
The list is thought to still be in use by fraudsters operating in the UK and abroad and is likely to have been circulated between different boiler room networks.
Boiler rooms usually contact people by telephone to con investors into buying non-tradable, overpriced or even non-existent shares. These fraudsters are unauthorised, normally overseas-based companies with fake UK addresses and phone lines routed abroad.
On average, victims of boiler rooms lose £20,000 each, costing the UK an estimated £200 million a year.
The FSA is writing to every single person on the list to alert them to their presence on it and to advise them how to avoid getting scammed. Anybody who thinks they may have been targeted by a boiler room scam should call the FSA's customer contact centre on 0845 606 1234.
This recent scam takes the total number of people contacted by the FSA about being a boiler room target in 2010 to 95,000.
"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," Margaret Cole, the FSA's managing director of enforcement and financial crime, said.
"Even if only one in ten we contact heed our warning it could mean around £96 million is not invested in these scams.
"Boiler room fraudsters often sound like the real deal so it's easy to be drawn in by their professional and high pressure sales tactics. In reality, however, the shares are worthless or don't exist and the money is lost forever."
People can avoid becoming victims of share fraud by:
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