Ex-Cabinet minister Clare Short has been formally reprimanded by Labour for saying she will campaign for a hung parliament at the next election.
Ms Short has said she is 'ashamed' of the Blair government
Chief whip Jacqui Smith has warned Ms Short she has breached the Parliamentary Labour Party's code of conduct, the BBC understands.
She was effectively wishing defeat on colleagues, it is argued.
Ms Short, the former international development secretary, could have been expelled from the party.
Labour's code of conduct calls for respect for fellow MPs.
But the Labour hierarchy decided anything stronger than a reprimand risked making her a political martyr, the BBC was told.
Last month Ms Short, who is stepping down as MP for Birmingham Ladywood at the next election, said: "I have reached a stage where I am profoundly ashamed of the government."
Ms Short resigned from the Cabinet on 2003 over the Iraq war.