There is not yet definitive proof that Wagner mercenary boss Yevgeny Prigozhin was onboard a plane that crashed with no survivors earlier this week but it is “highly likely” he is dead, Britain’s Ministry of Defense said on Friday.

Russian authorities have said Prigozhin was listed as a passenger on the plane which fell from the sky northwest of Moscow on Wednesday evening, two months to the day after he led an abortive mutiny against the army top brass.

Russian investigators have opened a probe into what happened, but have not officially confirmed the identities of the 10 bodies recovered from the wreckage.

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“There is not yet definitive proof that Prigozhin was onboard and he is known to exercise exceptional security measures. However, it is highly likely that he is indeed dead,” the British ministry said in a defense intelligence update posted on social media platform X, formerly known as Twitter.

Prime Minister Rishi Sunak said intelligence suggested Prigozhin was “most likely” on the plane.

“We’re obviously monitoring the situation very closely, working with our allies to establish what happened,” he told reporters.

The Ministry of Defense said Prigozhin’s demise would “almost certainly have a deeply destabilizing effect on the Wagner Group.”

“His personal attributes of hyper-activity, exceptional audacity, a drive for results and extreme brutality permeated Wagner and are unlikely to be matched by any successor,” it said.

“Wagner’s leadership vacuum would be compounded by the reports that founder and field commander Dimitry Utkin and logistics chief Valery Chekalov also died.”

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