Nearly 22,000 fraud victims are having their lost cash returned to them after officers cracked a mass marketing scam which could have yielded £35 million a year for criminals based overseas.
The Government's Serious Organised Crime Agency (Soca) intercepted payments responding to international lottery scams and bogus offers during a raid on an address in Somerset in June.
Each typically contained £20 cash with a total value of nearly half a million pounds.
Soca said the fraud could have netted more than £35 million in a year if the payments had reached their final destinations overseas.
Everyone who fell for the scam will receive a letter from Soca returning their money and warning them not to be taken in again.
Trevor Pearce, Soca's director of enforcement, said in the worst cases people can lose substantial amounts of money, even their life savings.
He 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.
"Unfortunately those who respond to a scam can find themselves on a list which is sold to other fraudsters."
Home Secretary Jacqui Smith said the operation sent "a very powerful message to criminal networks that we are working hard to stop them preying upon innocent people".
She continued: "It also serves as a reminder that people should be on their guard when being asked to part with their money in this way."
