Wessex Water has demanded answers from Royal Mail after copies of the utility company's customer magazine were posted to dozens of homes in the Bristol area with British National Party leaflets inside.
The company's customers complained of being angry and upset at finding political messages from the far-right party tucked inside the middle of the magazine.
Wessex Water spokesman Ian Drury said the company was not affiliated to any political party and added that it had made a formal complaint to Royal Mail.
He said: "We have received a handful of telephone calls and emails from customers who are angry and upset after they had received political information inside our customer magazine.
"The magazine is distributed by Royal Mail and should have been delivered on its own.
"We have asked Royal Mail to investigate the matter and explain why this has happened."
About 1.2 million copies of the magazine went out across the Wessex Water region last month, but it appears it was those living in Bristol and North Somerset who received the BNP mailshot.
Mr Drury said the magazines with the leaflets inside were posted to homes in Winscombe, near Weston-super-Mare, as well as to some areas of Bristol.
He added: "We are extremely proud of our magazine which we believe is one of the best customer publications produced by a utility company. We certainly wouldn't want its reputation to be affected any form of electioneering."
A spokeswoman for Royal Mail said political material might have been combined with the magazines to make it easier to post.
She said: "Royal Mail has a legal obligation under the Representation of People Act to deliver election material.
"It appears some leaflets may have been folded together with other mail simply for ease of posting through letterboxes.
"No materials should be combined with another customer's mailing and if this has happened we apologise."
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