This is very bad news for Gordon Brown.
This may seem like a strange thing to say, but in several conversations with Labour MPs and activists I have heard a version of the following: "We can't get rid of Gordon because Harriet would win the election to replace him."
With Harriet gone, the way is now clear for a genuine challenge.
The likelihood is that this won't happen. Although almost anyone would improve Labour's chances, the party is just exhausted (and this applies especially to the parliamentary party).
One Labour minister even told me this week that he hoped the Tories won the election definitively so that Labour could have the time to rebuild in opposition rather than fighting to undermine a tiny Conservative minority.