Royal Mail has been accused of allowing fraudulent letters to be sent en masse to vulnerable recipients.
Fraudsters are sending letters with Royal Mail branding to elderly Britons under its bulk mail contracts, the undercover investigation by The Daily Mail alleges.
The newspaper claims that the postal service has failed to crack down on the scam, despite being warned repeatedly for more than a decade.
The elderly victims, targeted by a self-proclaimed "mafia" who swap "suckers lists" of people, are said to lose billions each year in postal scams.
Scammers pay a company to print their letters in bulk, which are taken to the UK, and firms provide the Royal Mail with the letters which go into its system, it is alleged.
The letters are then delivered with the Royal Mail logo - which campaigners have warned gives them the seal of authenticity.
A Royal Mail spokesman said the service understood "the upset and disquiet this kind of mail can cause", but that it was illegal for it to open letters, and doing so would raise serious privacy concerns.
He said: "Just as newspapers would never intentionally publish advertisements that promote defective goods or illegal activities, Royal Mail never knowingly handles scam mail.
"We simply do not want to handle scam mail or make any money from this terrible activity. That is why we cease to do business with these people as soon as law enforcement agencies alert us to scamming activity."
The service said it has passed on the list of suspected companies supplied by The Daily Mail to its partners at the National Trading Standards' scams team for investigation.
Baroness Ros Altmann, former minister of state for pensions, said it was shocking that Royal Mail's "trusted brand" was being used to give credence to the scam letters.
She said: "Many have grown up believing the Royal Mail is an official body, part of government, and would assume something that has its apparent seal of approval is bona fide.
"It is vital that Royal Mail takes proper precautions to check the contents of bulk mailings, in order to help crack down on the scams that are causing so much misery to so many."
In a statement on its website, Whistl, one of the firms which provides the Royal Mail with mail to deliver, said it had no authority under the 2000 Postal Services Act to see its contents. Only the sender, the printer, mailing house and the recipient could do so, it said.
It added it had taken action to terminate relevant contracts when alerted to scam mail, but admitted the postal industry must do more.
Business Minister Margot James said she would be holding a meeting with Royal Mail, other postal operators and the National Trading Standards' scams team to ensure this issue was urgently tackled.