A scam letter scheme which could have netted an estimated £35m a year has been smashed by officers from the Serious Organised Crime Agency (Soca).
It follows a police raid on a Somerset address used as a front to operate the international scam.
Payments totalling nearly half a million pounds were recovered.
Cheques are now being returned to the 22,000 senders with a letter warning them about the scam and urging them to be more vigilant in future.
The letters, which were responding to international lottery scams and bogus offers, were intercepted by Soca during the raid in Somerset last June, before they could reach their final destinations overseas.
They typically contained £20 cash and Soca believes the fraud could have yielded over £35m a year if it had continued.
Trevor Pearce, Soca's director of enforcement, said: "Mass market frauds are often sophisticated and convincing - criminals know just which buttons to press to make people part with money, especially in an economic downturn when we are more susceptible to believing good news."
Don't be a willing victim for their crime - if something seems too good to be true, it probably is
People who respond to a scam can find themselves on a list which is sold to other fraudsters, and people can lose their life savings, he said.
"In this case we've been able to return people's money, and although the perpetrators are abroad and outside our jurisdiction, a substantial blow has been dealt to their operation by cutting off this particular channel," he said.
"The most powerful way to defeat the fraudsters though is to ignore them. Quite simply, be on your guard. Don't be a willing victim for their crime - if something seems too good to be true, it probably is."
Home Secretary Jacqui Smith congratulated Soca on the success of the operation, saying it sent a "strong message" to criminal networks that work was being done to stop them preying on innocent people.
She said: "We will continue to invest in agencies like Soca to enable them to tackle these criminals effectively. The public can be very confident that we are doing everything in our power to make it as difficult as possible for these groups to continue to operate."
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