A letter sent to residents urging them to check their eligibility to vote directed them to a hardcore porn website.
The reminder was supposed to direct recipients to a Mid Suffolk District Council website, but the address provided sends users to explicit content.
The letter also asked residents to call 00000 000 000 to confirm their voting eligibility.
A resident uncovered the error when he typed the address into his web browser.
The man who discovered the error, who did not wish to be named, said: "Quite humorous from my perspective but might give some older people a bit of a shock.
"I have informed them but I'm guessing a few have already been mailed out." Senior Director Mike Evans said: "It was not our intention to embarrass anybody in the district and the first thing I would like to say is to apologise to anybody who has received this letter."