UK regulators are in the process of contacting more than 75,000 investors to warn them that they may be targeted con artists after their names appeared on lists recovered from suspected boiler rooms.

The UK’s Financial Services Authority announced Tuesday that it has written to 76,732 people whose 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. The firms involved cannot be named due to ongoing legal action, the FSA says.

“All of the lists are believed to be current and were being used to either 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 FSA says.

The regulator says that this is the largest effort it has made to alert potential victims. The letters and emails from the FSA provide tips on how to spot a scam, avoid becoming a victim, and what victims should do if they have already invested. The FSA also has a team ready to answer questions about the letter and investment scams generally, and several high street banks have also provided numbers for their customers to contact.

Given the large number of names on the list and the high volume of expected incoming calls to its contact centre, the FSA will be sending the letters and emails out in waves. The first 10,000 letters will arrive today and a further 10,000 letters will be sent each week. The first 5,000 emails will be sent on April 30, with a further 5,000 each week.

“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,” said Jonathan Phelan, the FSA’s head of unauthorised business.

“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,” he added.