Sir Keir Starmer is now "more likely" to face a leadership challenge and the briefing war between Number 10 and Wes Streeting may have "pulled the plug" on his time office, Harriet Harman has said.
Speaking to Sky News' Electoral Dysfunction podcast with political editor Beth Rigby, the Labour peer said there was "still time" for Downing Street to "get their act together".
But she warned Labour MPs had been left "dismayed" by the "unforced errors coming out of Number 10".
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Baroness Harman's comments come in the wake of a major row sparked by "allies" of the prime minister briefing journalists that he was facing a leadership challenge that he would fight, and named Wes Streeting as a key plotter.
The health secretary was forced to deny the accusations - which Sir Keir later said were "never authorised" by him - and told Sky News allies of the prime minister had tried to "kneecap" him.
Baroness Harman said the situation is "a mess of Number 10's making", adding: "If the intention was to strengthen Keir and weaken Wes, it has had exactly the opposite effect."
Could Starmer be gone next year?
It has been suggested Sir Keir could face a leadership challenge if Labour perform poorly at local, Holyrood, and Senedd elections in May.
"They have got time to get their act together," Baroness Harman suggested. "But I think if there is a challenge after May, what has happened this week makes the challenge more likely.
"And I think Keir Starmer might well be looking back on this week and thinking that was the thing that pulled the plug."
Labour MPs have been vocal in expressing their anger with the briefing, and Baroness Harman said even Starmer loyalists are frustrated.
"The faithfuls are losing faith at the moment because all they can see is unforced errors," she said.
"And also, the golden rule is if you're going to start some sort of story running, that is risky, you don't do it on a Tuesday: the day before Prime Minister's Questions."
'Number 10 failed either way'
The Labour peer continued by saying that either the briefing was a serious plan from Number 10 that "went incredibly badly wrong", or they "failed" to "kill the story".
"So Number 10 failed either way, because they promoted a story they shouldn't have been, or failed to stop a story."
"The prime minister should never be a destabilising force and should never be disloyal to cabinet ministers," she added.
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