Mr Coleman had denied breaching Barnet Council's code of conduct
The mayor of a London borough breached the councillors' code of conduct in his dealings with a constituent, an internal investigation has found.
Brian Coleman received an e-mail from blogger Roger Tichborne, critical of the leadership of Barnet Council, which was sent to all local Tory councillors.
In a reply he called Mr Tichborne "an obsessive, poisonous individual".
The blogger argued this was improper behaviour for a councillor and Barnet's standards sub-committee has agreed.
The three-member panel will release the full reasons behind the ruling at a later date.
It is unclear whether Mr Coleman will face any punishment.
'Addicted to cash'
The code of conduct for councillors states that "you must treat others with respect".
"You must not conduct yourself in a manner which could reasonably be regarded as bringing your office or authority into disrepute," it adds.
Mr Coleman used the e-mail to tell Mr Tichborne: "You are an obsessive, poisonous individual and Labour Party member whose blog is full of lies, half-truths and misinformation.
You can tell someone you think they're an idiot, you think they're a liar. You can do all that without insulting someone
Roger Tichborne Blogger
"Do not e-mail me again."
Melanie Carter, an independent lawyer who investigated the case, told the hearing that the message was "abusive and offensive", and showed disrespect for a member of the public who had made a complaint.
Mr Coleman, who denied breaching the code of conduct, said the blogger had written abusive and provocative remarks about him on several previous occasions, once describing him as being "addicted to cash and hospitality".
But Ms Carter said: "Even if his previous blogs had contained sustained abuse and lies, this would not justify this response."
After the result was announced, Mr Tichborne said Mr Coleman should have been able to "address any criticism he had of me in a civil manner".
"You can tell someone you think they're an idiot, you think they're a liar," he told BBC London.
"You can do all that without insulting someone."
As well as being mayor of Barnet, Mr Coleman is a London Assembly member and the chair of London's Fire Authority.
It is unclear whether Mr Coleman will face any punishment
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