SOME 76,000 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 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. The FSA is also sending email warnings to more than 19,000 people on the lists whose addresses were not recovered.

The letters are written to people the FSA "believe may be targeted by fraudsters offering unauthorised investment services". They warn potential victims to "stay alert" even if they have not already been offered unsolicited advice, as lists are often traded between fraudsters, meaning they could be contacted in the future.

It is the largest number of targeted people the FSA has contacted in one go, and several high street banks, including the Bank of Scotland and Royal Bank of Scotland, have joined forces with the authority to provide phone numbers for customers to call if they are concerned they may have fallen victim to fraudulent schemes. Coutts, Halifax and Lloyds TSB are also backing the mailshot with a phone number for their clients.

Jonathan Phelan, the FSA's head of unauthorised business, said: "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 a 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 said recipients should be aware 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 that the FSA believes was operating an unauthorised business, which it said 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 to ever happen, the regulator said.

Mr Phelan said: "These lists are nothing more than fraudsters' phone books, and the people who use them are ruthless, calculating 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 said people who have been contacted by a firm offering to buy or sell investments should be especially wary if they have been cold-called. They should check the status of the firm with the FSA and call 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 to 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.

The FSA has set up a team to deal with questions about its letters as well as investment scams generally. It can be contacted on 0845 155 6355.