The Royal Mail has started an inquiry after a letter, which appeared to have been delivered by a postal worker, warned against "government propaganda".
A couple in Romsey, Hampshire, received the letter, titled "message from your postman", detailing which media outlets they should trust.
The note, received in October, also offered additional help to those stuck at home and tips on avoiding junk mail.
The Royal Mail said it had not written or endorsed the letter.
The letter headlined one section: "Real news, definitely not government propaganda."
It said: "If you want to find out what is really happening, stop using BBC, ITV, C4, C5."
The author then recommended a Talk Radio show and Telegraph podcast as preferable alternatives.
It is not known how many copies of the letters were delivered or if any complaints were made, despite a request to Royal Mail.
Royal Mail did, however, describe it as an "isolated incident" and confirmed it had launched an investigation.
A spokesperson said: "While we always welcome our postmen and postwomen going the extra mile to try and help their customers, this letter was not endorsed or written by Royal Mail.
"We apologise to impacted customers for any confusion caused."
