Ms James appeared on Sky News to analyse their performances in our Battle for Number 10 programme last night - where the pair faced a separate grilling each from Kay Burley.

She welcomed the format of the show for allowing viewers to see the contenders in "human form", praising Mr Sunak's "optimistic energy" and Ms Truss's "calm, stateswoman-like" qualities.

But she said: "They both showed moments of weakness, they both showed moments of flaws."

Ms James said the foreign secretary is like an opera singer that "cracks on the highest notes" when she doesn't like a question - while the former chancellor sticks out his tongue "like a baby rejecting food".

She said of Ms Truss: "When she gets under pressure, she's a bit like an opera singer that cracks on the highest notes.

"We get this staccato blink rate, her vocal tone starts to crack slightly, it becomes rather brittle and she very, very patently shows that she doesn't like the questions or that she's been put under pressure."

She revealed another subtle clue for spotting when Ms Truss isn't happy about a question, saying her jaw "goes from side to side" and "the lower lip gets sucked in".

"These in poker will be tells that she's not comfortable with a question," Ms James said.

However, she said Ms Truss had "grown in confidence" since the start of the leadership contest, adding: "She's becoming more heavyweight with every appearance that she makes."

On Mr Sunak, the body language expert noticed he poked out his tongue every time he was asked a difficult question.

She said: "When he gets those killer questions, out comes the tongue. It's a relic gesture of a baby rejecting food. It's usually a gesture of rejection and disgust.

"And I don't think he was disgusted by Kay at all. It was her questioning that he was clearly not happy with."

She said leadership hopeful was often guilty of "overkill gestures", comparing his body language at times to a "car salesman".

However, she suggested this may be why he won over the audience of mostly undecided voters, who deemed he had performed better than his rival, despite her being consistently ahead of him in the polls.

She noted his use of the "thumb of power" drew him a large round of applause when talking about beating Labour leader Sir Keir Starmer at a general election.

Ms James said: "His verbal message is about caution and yet his body language message looks like optimism, energy, enthusiasm. He walks around the stage, he is the one that employs the most energy while he is talking.

"So that kind of counterintuitive message might have had a very positive effect on the audience."

Read more:
What did Truss and Sunak promise during the Sky News Battle for Number 10?

'The invisible brick'

While both contenders have their individual strengths and weaknesses, Ms James said they shared one common body language trait - the "invisible brick".

She said: "This was one gesture that they both had in common, they used it virtually throughout.

"This suggests that you've got a full grip on the problems ahead. You know what the problems are going to be. You're holding them like an invisible brick."

Subscribe to the Daily podcast on Apple Podcasts,  Google Podcasts,  Spotify, Spreaker

However, she said what worried her was that this was not followed up by "solution body language".

She said: "It would be great to see more of those precision gestures that would lead me to think that they're not just aware of the problems.

"They also know exactly and perhaps financially, forensically, they know exactly what they're going to do."