COUNCIL leader Kath Pinnock has been found guilty by an internal inquiry of an inappropriate use of council postage.
An investigation was sparked by Tory councillor Mark Hemingway after receiving a letter from the leader.
Opposition parties said the letter was produced and posted using council resources and detailed the Liberal Democrat policies on health.
The inquiry, undertaken by chief executive Robert Vincent, judged the letter to have been promoting the Liberal Democrats.
Council rules state it is inappropriate for councillors to use council resources and postage to promote a political party.
The Liberal Democrats admitted sending out 168 similar letters.
Conservative leader Clr Robert Light said: "What Clr Pinnock has done has brought the council into disrepute.
"What is of greater concern is that unless this letter had not fallen into the hands of someone who knew it was wrong to send it out at the council's expense, it would have gone undetected."
Labour leader Clr Mehboob Khan said this was not the first time the Lib Dems had "abused" council resources.
He added: "The council must revise its rules to stop this being possible in the future."
But Clr Pinnock said the issue was nothing but "electioneering".
She added: "I hold my hands up and say that a mistake was made.
"The letters were talking about the threat to services at the HRI and they started with the words "Thanks for supporting our campaign".
"However the council doesn't campaign and in retrospect it should have said "Thanks for supporting our petition."
"The chief executive called me up and asked me about the issue and I offered to pay the money back
"I paid back just under £50 which covered the stamps, paper and even the time taken by council staff to produce the letters."