The FSA wrote to the shareholders after acquiring the fraud database with personal details including names, telephone numbers and addresses from Canadian authorities.
The regulator says it is likely that the list has been sold to a number of share fraud gangs.
Share fraudsters are often based overseas and use high pressure sales techniques to target investors illegally, offering them non-tradable, overpriced or even non-existent shares.
The FSA is urging shareholders to check that anyone offering to sell them shares is registered with the FSA and to report any company that cold calls them to sell shares.
Head of retail enforcement Jonathan Phelan says: “This is a great example of how international co-operation can help protect people from falling victims to share fraud. The details on the database provide fraudsters with valuable information that can be used to convince people that they are dealing with legitimate stockbrokers, in order to win their trust.
“These criminals sound authentic, are smooth talkers and can be very persistent. If anyone calls you out of the blue offering to sell shares, just hang up or you stand to lose a lot of money with very little hope of ever getting it back.”