Prime Minister appeared to be feeling the pressure while facing off with Rishi Sunak at PMQs.
In their first Commons clash since Parliament returned from summer recess, at least four times. Body language expert Judi James believes the slip-up is a clear sign that Sir Keir is "letting the job get on top of him" as he wrestles with growing fury against the Labour party.
She told Express.co.uk: "The lighting in the House is not the most flattering but Starmer's face looks riddled with anxiety here, creating a strong wrinkling of his brow.
"He seems to enunciate the words 'Prime Minister' poorly, hinting that he might have been worried about repeating his earlier mistake but he shows no sign of recognising that he has."
Judi continued: "He even leans forward onto his elbow in a pale imitation of the kind of cocky attacks party leaders tend to inflict on each other during PMQs and his head literally does cock slightly, but it's a very depleted-energy-looking performance with this being something of a telling low point."
Suggesting where Sir Keir needs to improve, Judi added: "Starmer needs to learn the kind of charisma and confidence that will allow him to recover from slips and trips like this.
"They can actually be ice-breakers for a nimble speaker but keeping this kind of straight face and trying to ignore them, as Starmer does, will leave him vulnerable to ridicule and accusations of allowing the job to get on top of him.
During PMQs, Mr Sunak ripped into Sir Keir's decision to slash winter fuel payments for millions of pensioners. The Prime Minister opened the session in the Commons by highlighting bumper pay offers made by the new Government in a bid to settle train strikes.
Mr Sunak asked: "Why did he choose train drivers over Britain's vulnerable pensioners?"
But Sir Keir hit back, insisting he had to "clear up the mess left by the party opposite" as he repeated Labour's claim about a £22 billion black hole in the public finances.
as he asked why the Prime Minister halted implementation of a system for monitoring interference from foreign states.
Mr Sunak told the Commons: "The last government also established a new system of registration and monitoring to protect the UK from interference from foreign states, including China, and Iran.
Mr Sunak said: "It's called the foreign influence registration scheme, it was described as essential by MI5 in the fight to help keep Britain safe. But since the Prime Minister took office, he has halted its implementation. Why?"
Sir Keir responded: "That isn't correct." To which the Conservative MP hit back: "Well, Mr Speaker that is very clearly what the Government has said.
"Only last week he said at the Despatch Box that he would give the security forces the powers they need and if he is going to fulfil that promise, I would urge him to get up to speed on this issue and therefore implement the scheme."
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