THE Mayor of Barnet breached the council code of conduct by calling a blogger an "obsessive, poisonous individual" in an email, according to an officer who investigated the case.
Brian Coleman could be suspended without pay from Barnet Council if found guilty of misconduct at a standards sub-committee meeting on Thursday.
According to the monitoring officer's report, Mr Coleman's March 5 email to Mill Hill musician Roger Tichborne, 47, failed to comply with Paragraph 3 of the code: failing to treat others with respect.
A committee will decide whether it agrees with the findings, written by Melanie Carter, of Bates, Wells and Braithwaite, who was commissioned to write the report, on Thursday.
The Barnet councillor and assembly member would also be liable for £10,000 in legal fees, paid to Beachcroft Ltd - the top law firm he requested after rejecting the service provided by the council insurers.
Mr Tichborne - who has a blog called The Barnet Eye - made his complaint on March 6 after Mr Coleman replied to an email he sent to all Tory councillors.
Mr Tichborne's email complained about comments made by council leader Mike Freer on his Leader Listens website, where he described local bloggers as “nutters".
Replying to the same list of councillors, Mr Coleman wrote: "You are an obsessive, poisonous individual and Labour party member whose blog is full of lies, half truths and misinformation, do not email me again."
Mr Coleman claimed his email was a justified and "truthful" response to a sustained political campaign launched against him by Mr Tichborne, and was a fair use of free speech.
He argued Mr Tichborne had already set the tone of the debate with his highly insulting blogs, which referred to the London fire authority chairman as "arrogant, lazy and useless", "Barnet's favourite numpty" and "addicted to cash and hospitality".
The report rejected these arguments, concluding Mr Tichborne was not a political opponent and that Mr Coleman's tirade amounted to a personal, not political, attack.
The officer said freedom of speech was not absolute and concluded that Mr Coleman had "overstepped the normal limits of expression for an elected member".
The report states: "This was not a matter of political or public importance. Councillor Coleman was not representing his consituents or seeking to further the interests of the council.
"He was, in my view, using the email exchange to 'get back' at an individual member of the public who he, rightly or wrongly, perceived as having abused him on a personal basis."
Mr Coleman has attracted controversy over his excessive taxi fares after claiming more than £20,000 over three years - more than the rest of the London Assembly put together.
He has also attracted the ire of the London Fire Brigades' Union, who resent his allegedly "hypocritical" demand for cutbacks.
Mr Tichborne, from Millway, said: "I have every faith in the committee to make the right decision here.
"If he is found to have breached the code, I hope he learns something from whatever punishment is meted out to him.
"The committee has an excellent opportunity to help him learn something - though whether he will or not, I don't know.
"I think if Brian Coleman could just treat people with a bit of respect and be more polite, it could do him a lot of good."
The Barnet standards sub-commitee will meet on Thursday at Hendon Town Hall, in The Burroughs, at 10am.
It does not have to accept the findings of the monitoring officer's report and will decide independently, on a balance of probabilities, whether a breach of the code has occurred.