Jeremy Corbyn's supporters suspected Sadiq Khan of being an opponent for a long time. So when the London Mayor openly attacked the Labour leader they were ready to hit back.
Their criticism focused on how Mr Khan had been happy to accept Mr Corbyn's backing, not to mention support from the leader's army of Momentum activists, in his bid for power in London.
General secretary of the Fire Brigades Union Matt Wrack, a regular speaker at Momentum rallies, was among those who spoke out after Mr Khan branded the Corbyn leadership a failure.
He said: “Sadiq comes from that part of the Labour Party that was in government under Blair and Brown.
"It is disappointing nevertheless. Jeremy Corbyn gave full support for Sadiq in his campaign for mayor, as did a number of unions including my own.
"Sadiq hasn't consulted us or Labour Party members across London about this latest statement. I think a lot of people will be quite let down by that."
Mr Khan had kept a trappist-like silence over who he would back in the leadership contest saying only, when asked in a recent interview who he was for, that he was “for London”.
But there were clues to his direction of travel, in particular his insistence that winning elections was paramount. Mr Corbyn’s opponents constantly claim the leader is unelectable.
Momentum activists campaigned for Mr Khan in London during the election earlier this year, despite many raising concerns in branch meetings that he did not share their politics.
Senior Momentum officials are now working on Mr Corbyn’s bid to get re-elected as Labour leader.
A spokesman for the Jeremy for Labour campaign highlighted how Mr Corbyn had even defended Mr Khan in the Commons when David Cameron accused him of having links with extremists.
"Sadiq Khan is entitled to his opinion," the spokesman said.
"But he won the London mayoralty, as others have pointed out, by standing on a Labour platform under the leadership of Jeremy Corbyn backed by the huge numbers who have joined our party to support Jeremy and now campaign for Labour."
In his piece for the Observer, Mr Khan said his leader did not have the qualities needed to take the party to success and so he was backing rival Owen Smith.
He wrote: "By every available measure, if Jeremy remains as leader, Labour is extremely unlikely to win the next general election.
"The hopes of the members who have joined our party would be dashed again. Jeremy has already proved that he is unable to organise an effective team, and has failed to win the trust and respect of the British people.
"Jeremy's personal ratings are the worst of any opposition leader on record - and the Labour party is suffering badly as a result. He has lost the confidence of more than 80% of Labour's MPs in Parliament - and I am afraid we simply cannot afford to go on like this."
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