LONDON (Reuters) -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 Defence 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.