Nearly 80,000 people who feature in fraudsters' "phone books" are to be contacted by the City watchdog in the biggest operation of its kind.
The 76,732 names have been drawn from various "sucker lists" held by companies which the Financial Services Authority (FSA) believes were selling investments in land or shares that turned out to be non-existent or worthless.
Letters from the regulator will arrive at the addresses of those on the list over the next six weeks as part of a campaign known as Operation Bexley, and the FSA is also sending email warnings to just over 19,000 people on the list whose addresses were not recovered.
The letters warn potential victims to "stay alert" even if they have not already been offered unsolicited investment advice, as lists are often traded between fraudsters, meaning they could be contacted in the future.
Combined into one list, this is the largest number of targeted people that the FSA has contacted in one go.
Jonathan Phelan, the FSA's head of unauthorised business, says: "If you get a letter or email from the FSA over the next five or six weeks, please read it – it could save you tens of thousands of pounds.
"If you have already been contacted by a firm offering you a 'once in lifetime' investment opportunity or have already invested, then tell us. The information you have could help us catch criminals and shut down their scams."
The FSA says recipients should be aware that the watchdog will not call them for more information or ask for money, bank account or personal details.
The largest list was recovered from the premises of a firm the FSA believes was operating an unauthorised business, which it says cannot be named due to continuing legal action.
All of the lists are believed to be current and were being used either to sell fake or worthless shares, or plots of land with the promise of great investment returns once developed, even though this was unlikely ever to happen, the regulator says.
Phelan says: "These lists are nothing more than fraudsters' phone books and the people that use them are ruthless, calculated and will stop at nothing to steal your money.
"A call out of the blue is one of the hallmarks of investment scams, so if you ever get an unexpected call with promises of fantastic returns, you should be extremely sceptical."
The FSA says people who have been contacted by a firm offering to buy or sell investments should be especially wary if they have been "cold-called" out of the blue.
They should check the status of the firm with the FSA and call them back on the switchboard number provided by the watchdog.
Unauthorised firms are not covered by the Financial Services Compensation Scheme and if someone invests through such a business, it is likely they will lose their money if the firm goes bust or disappears.
The FSA has recently issued warnings about "boiler room" fraud, whereby unauthorised overseas companies with bogus UK addresses cold-call people and try to pressure them into buying non-tradable, overpriced or even non-existent shares.
It has also highlighted land banking companies, which divide land into smaller plots to sell to investors on the basis that once it is available for development, the plot will soar in value – but the land often has little chance of being built on.
Such schemes also use unsolicited calls and high pressure to convince people to invest.
The FSA has set up a team to deal with questions about its letters as well as investment scams generally, and it can be contacted on 0845 155 6355.
Several high street banks have also provided phone numbers for their customers to call if they are concerned and callers should quote 'Operation Bexley'.