Investors have been warned to be wary of cold calls from share swindlers after financial regulators discovered a "hit list" of 38,000 people being circulated by foreign fraudsters.
The names and addresses were on a master list of "suckers" used by "boiler rooms", a financial racket in which worthless shares are sold to trusting individuals.
Most of the potential victims live in London and the South East, but significant numbers are based in Yorkshire and Lancashire, said the Financial Services Authority (FSA).
The regulator is contacting the individuals to warn them they may be phoned by unauthorised "stockbrokers" seeking investors. It said it was the largest boiler room list it had recovered, indicating that the swindlers were becoming more active during the recession.
Under the scam, which has been in existence for several years, sales people phone householders out of the blue seeking funds for what are claimed to be lucrative businesses. Although they are usually based abroad, the swindlers often use fake UK addresses and phone lines. They employ high-pressure sales tactics and hope for quick gains.
After handing over the money the hapless "investors" find that the shares they have bought are worthless. As the firms are not regulated by the FSA, they cannot get their money back from the Financial Ombudsman Service and Financial Services Compensation Scheme.
Men over the age of 40 who have some experience of share dealing are particularly likely to fall for the fraud, according to the FSA.
Share frauds are estimated to cost British people £200m a year, and the number of boiler room scams appears to have increased sharply recently.
Last year the FSA received calls from 3,100 people who had been contacted by boiler rooms, of which 734 had been successfully conned. Victims lost an average of £24,000 each.
In February, the FSA reported that it had seen a dramatic increase in the number of fraudsters selling shares using the names, registration numbers and addresses of authorised firms in an attempt to trick people into thinking they were legitimate.
One victim, Jimmy Gill, a London pharmacist, told the BBC he had lost £40,000 which he thought was going into genuine shares.
"They were very persistent," he said. "They lull you into a false sense of security. The first transaction was for only £5,000, a dry run, then the big one was for £35,000."
The FSA found a master list of 10,000 potential victims in February. Authorities in the United States handed over this latest list, which contains 38,242 names and addresses.
The potential victims are being sent a letter from the FSA headed: "This is a warning – you may be targeted by fraudsters." It advises recipients not to take up offers from cold calls of "once in a lifetime chances" or "investment opportunities" because "you could lose a lot of money". Anyone contacted by a potential boiler room should contact the City of London Police on 0845 602 2185.
Jonathan Phelan, head of the unauthorised business department at the FSA, said: "This is the biggest list we've ever recovered and by acting quickly and contacting every single person on it we're hoping we can stop people losing money.
"Boiler room fraudsters often sound professional so it's easy to be drawn in by their overblown claims and give them money to invest. The reality is, however, that the shares are worthless or don't exist and the money is lost forever. Cold calling to sell shares is rarely allowed, so anybody who receives a call like this should be extremely sceptical."
The FSA advises anyone who receives a phone call about buying shares out of the blue to simply hang up. People thinking of buying shares should first check that the company is authorised by the FSA and then call them back, using the contact details provided in the FSA register.
Anyone contacted by a suspicious firm or who has doubts about one should report the incident to the FSA as soon as possible on 0845 606 1234, the regulator added.