Seven bags of mail that should have been delivered by TNT Post have been found discarded in green spaces in Eastcote over the past two weeks.
Volunteers belonging to the Friends of Eastcote House Gardens say they have been discovering the bags during their conservation and gardening sessions in Long Meadow and Forge Green.
Eastcote Conservation Panel and Friends of Eastcote House Gardens chair Lesley Crowcroft said the bags were left in a wet, messy condition.
The Friends group handed the mail to Hillingdon Council after finding them in areas surrounding the gardens in High Road Eastcote.
In a letter to the Uxbridge Gazette, Mrs Crowcroft wrote: "How much more mail in other areas is suffering the same fate? Is it being handed in to the authorities or are the unscrupulous gaining access to your private affairs?
"Many Credit Card companies, Banks, Insurance Companies use TNT mail, this means your private and important documents are being carelessly abandoned by the employees of TNT.
"It was a Government decision to give the Royal Mail competition, which appears to have been a very unsuccessful idea.
"We can take action, write to all companies with whom you have dealings, who sent your important documents via the TNT service, complaining about the service, also inform your MP asking for them to take action."
TNT Post - which describes itself as Royal Mail's number one challenger for business post and business mail of all kinds - has said police are investigating.
