From Laura Kuenssberg:
Damian McBride denies briefing against Darling.
If we accept that at face value, then there are only two possibilities, Ed Balls and Charlie Whelan.
If it transpires that it was Balls or a combination of both, that could well end his leadership ambitions.
Darling’s outburst could not only determine the budget strategy, but also who leads the Labour party after the election.