Birmingham MP Clare Short has received a written reprimand after announcing that she hopes Labour fails to win the next election.
The decision, by the Government Chief Whip, Redditch MP Jacqui Smith, ends any possibility that Ms Short will be thrown out of the party.
Instead, party chiefs have settled for sending her a stiff letter asking her to behave in future.
Ms Short has already confirmed she plans to stand down from the Commons and will not stand again in her Ladywood constituency.
The reprimand follows her comments that she would back a hung Parliament at the next election, because it would give MPs more power to hold the Government to account.
This could only happen if Labour loses its overall majority - in other words, if it lost some of the seats it currently holds.
Although Ms Short is used to being at the centre of controversy, the latest comments caused particular outrage among Labour backbenchers.
They were condemned as "completely unacceptable" by Ms Smith, and there was speculation Ms Short could lose the party whip, effectively expelling her from the Parliamentary Labour Party.
But yesterday Ms Smith told a committee of Labour backbenchers that she had settled for sending her a written reprimand instead.
The letter tells Ms Short's her comments were unnaceptable and warns her not to repeat them.
However, there is no suggestion what might happen to her if she refuses to toe the line.
A copy of the reprimand has also been sent to the Ladywood constituency Labour Party.
A Labour source said: "Clare Short received a written reprimand from the Chief Whip. That was reported to the Parliamentary Committee who fully supported her action."
If Ms Short is involved in further controversy the Chief Whip could take a fresh look at possible action against her.
However, the feeling in the top levels of the Labour Party is that there is little that can be done as she is planning to resign anyway.
The process of choosing a new candidate for Ladywood has not yet begun but Labour is to impose an all-women shortlist.
