The Mayor of London, Ken Livingstone, has been suspended from office on full pay for four weeks for comparing a Jewish journalist to a concentration camp guard.
Mr Livingstone, 60, immediately criticised the panel's ruling as one that 'strikes at the heart of democracy', and said he is considering an appeal.
The Adjudication Panel for England, an independent tribunal which hears complaints against local authority members, ruled that Mr Livingstone had brought his office into disrepute.
The panel said his remarks to Oliver Finegold, a reporter for the London Evening Standard, on 8 February 2005 had been 'unnecessarily insensitive and offensive'.
Some members of the Greater London Authority have described the ruling by the unelected body as an 'hysterical overreaction'.
Mr Livingstone claimed the jibes were triggered by his dislike of Associated Newspapers, which owns the Evening Standard and its sister national paper, the Daily Mail.
He argued they referred to the Daily Mail's reputed support for fascism and opposition to Jewish refugees in the 1930s.
The London Evening Standard's editor, Veronica Wadley, said the mayor's behaviour was disappointing and had disgraced himself and his office.
The ban is due to begin on Wednesday.