A Childline leaflet referring to child abuse broke advertising rules by failing to make clear it was marketing material, a watchdog has ruled.
The Childline number is 0800 1111
The direct mailing risked causing fear and distress to people, the Advertising Standards Authority (ASA) said.
Written in the form of a mock log sheet by a counsellor, it detailed calls to the children's helpline.
The NSPCC, which runs Childline, said it would amend the envelope and mark it clearly as being from the charity.
'Fear and distress'
Three people complained to the ASA while the NSPCC received 143 complaints about the mailing which was delivered to 200,000 households.
Following an investigation, the ASA said the leaflet broke advertising rules on truthfulness, fear and distress, and identifying marketing material.
"We noted the letter did not make clear the calls on the counsellor log sheet were not genuine but considered that, even if it did, the log sheet could still mislead and cause fear and distress if it was read before the letter," it said.
One of the calls on the pretend log sheet referred to child abuse while another was about domestic violence.
The log sheet, which came with a letter asking for donations, said many youngsters could not get through to Childline when they tried to call.