The FSA says it is contacting 76,732 people to let them know they are targets for conmen.

The regulator says the names appeared on a number of lists recovered from companies that it believes were fraudulently selling investments in land or worthless, sometimes non-existent, shares.

Combined into one list, this is the largest number of target victims that the FSA has ever contacted in one go.

Letters from the regulator will be arriving from today. The FSA says that most of the list contains the names and addresses of the targets, but in 19,101 cases only email addresses are listed, therefore the FSA will be sending those people an email warning.

The letter, which can be found online, provides tips on how to spot a scam, avoid becoming a victim and what to do if you have already invested. Recipients should be aware that the FSA will not call them for further information and will never ask for money, bank account or personal details.

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 you 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.”