The Financial Services Authority (FSA) is contacting more than 75,000 people warning them they are being targeted by financial conmen in what is the largest number of potential victims it has contacted in one go.
The watchdog said it is writing to 76,732 people to inform them fraudsters are trying to con them out of money.
The targeted people were found on a number of lists recovered from companies the FSA believes were fraudulently selling investments in land or worthless, sometimes non-existent, shares.
When combined into one list, it represents the largest number of target victims the watchdog has contacted in one batch.
Targeted individuals will receive warning letters or emails from the regulator from today, the FSA said.
The letters provide tips on how to spot a scam, avoid being a victim and what to do if already invested.
"If you have already been contacted by a firm offering you a ‘once in lifetime' investment opportunity or have already invested, then tell us," said FSA head of unauthorised business Jonathan Phelan . "The information you have could help us catch criminals and shut down their scams."
Putting the tech into protection
Square Mile’s series of informal interviews
Fallout from Haywood suspension
Launching later in 2019
£80bn funds under calculation