The Financial Services Authority (FSA) says it has found the biggest ever "suckers" list of potential targets for share fraudsters.
The regulator is writing to more than 38,000 people whose names, addresses and telephone numbers are on the list.
They are being warned that their details are still being passed around, so they could be contacted at any time.
The information is bought and sold by "boiler rooms", teams of bogus dealers who try to sell worthless shares.
"The greatest concentrations of targets are based largely in London and the South East, but there are significant numbers present in Yorkshire and Lancashire too," the FSA.
"They were very persistent," laments Jimmy Gill, a London pharmacist who has already been ripped off by a boiler room.
Mr Gill told the BBC he had lost £40,000 which he thought was going into genuine shares.
"They lull you into a false sense of security," he explains.
"The first transaction was for only £5,000, a dry run, then the big one was for £35,000."
The FSA believes UK investors lose hundreds of millions of pounds a year to boiler rooms, with the average loss per investor being put at £20,000.
Boiler room victims are often men, aged over 40, who have already had some experience of share dealing.
"We had several hundred callers last year saying they were victims of boiler room scams," said Jonathan Phelan of the FSA.
"A proper FSA authorised firm should not be cold-calling you - you should not get a call out of the blue.
"The advice is hang up on them," Mr Phelan added.
How they work
Boiler rooms work outside the law and usually outside the UK.
The fraudsters tend to employ young dealers from the UK, often straight out of school, making them work the telephones using the hit lists.
They add in information about investors' interests, the cars they drive and any genuine shares they hold, to provide conversational gambits for the future.
Victims have little chance a getting help from the Financial Services Compensation Scheme.
This covers losses on investments sold by firms which are authorised to operate in the UK, but which become insolvent.
Once a potential victim has been duped, he or she will be contacted repeatedly by "loaders", experienced scam merchants looking for repeat sales, and "sloppers" who promise that the lost funds can be retrieved if further payments are made.
The trickery is backed up by sophisticated selling techniques gleaned from the world of legal marketing.
Jimmy Gill says he was "dumbstruck" when he found that he couldn't get hold of the firm which had taken his money. Its website had disappeared.
"I would consider myself prudent and I've invested in shares before," he says.
"If I can be caught out I'm sure a lot of other people can be."
The FSA's mass mailout is being called "Operation Domingo".