Police have seized thousands of scam letters aimed at luring potential British victims into handing over cash to fraudsters.
Investigators said the items are designed to encourage people to invest in fraudulent schemes linked to lotteries, shares and inheritance claims.
Overseas crooks attempt to persuade victims to transfer money to them with promises of valuable goods or bigger payouts that are never handed over.
Officers at Scotland Yard's economic and specialist crime command intercepted the mailshot as part of a long-running inquiry. It was the first time police have moved to stem the flow of fraudulent junk mail before it reaches the letterboxes of potential victims.
Police also shut down five rented post-boxes which are used to dupe victims into believing the companies are reliable and based in London.
Those who reply to the first letter, known as a "tempter", are then added to so-called "sucker" lists which are traded among criminals. Victims are often vulnerable people, including the elderly and those suffering illnesses such as dementia.
Police believe up to £3.5 billion is stolen from Britons every year, with as much as £2.4 billion being lost as a result of mail scams.
Detective Superintendent Mark Ponting, of the Metropolitan Police, said: "The individuals behind this type of crime are cynical and pernicious, making their living by targeting and exploiting some of the most vulnerable and needy people in our society."
The investigation began last year when detectives contacted 11 scam mail victims from across Britain who had been repeatedly targeted.
They were asked to collect their post and pass it to police, who traced the letters back to their source with the help of Royal Mail and Spring Global Mail. The two companies agreed to alert police when the next shipment of post linked to the same source arrived in Britain.