An armed rebellion led by the Wagner mercenary group marks “the most significant challenge to the Russian state” in recent times, according to UK defence officials.

Russian President Vladimir Putin called the actions of the private military company a “betrayal”, after its chief Yevgeny Prigozhin vowed to topple Moscow’s military leadership.

In its latest intelligence briefing, the Ministry of Defence (MoD) said that “over the coming hours, the loyalty of Russia’s security forces… will be key to how the crisis plays out” as Wagner forces appear to be moving towards the Russian capital.

The department tweeted that the feud between the Wagner group and the Russian defence ministry “escalated into outright military confrontation” in the early hours of Saturday.

“In an operation characterised by Prigozhin as a ‘march for freedom’, Wagner Group forces crossed from occupied Ukraine into Russia in at least two locations,” the MoD said.

It said that in the southern Russian city of Rostov-on-Don, “Wagner has almost certainly occupied key security sites”, including the Russian military headquarters that oversees the fighting in Ukraine.

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Yevgeny PrigozhinYevgeny Prigozhin, the owner of the Wagner group military company, released video from the southern Russian city of Rostov-on-Don (Prigozhin Press Service via AP)

“Further Wagner units are moving north through Vorenezh Oblast, almost certainly aiming to get to Moscow. With very limited evidence of fighting between Wagner and Russian security forces, some have likely remained passive, acquiescing to Wagner.

“Over the coming hours, the loyalty of Russia’s security forces, and especially the Russian National Guard, will be key to how the crisis plays out.

“This represents the most significant challenge to the Russian state in recent times.”

Wagner troops have played a key role in Russia’s invasion of Ukraine, succeeding in taking Bakhmut, the city that has seen the bloodiest and longest battles.

But Mr Prigozhin has stepped up his criticism of Russia’s military leadership, accusing it of shelling his fighters.

The escalation of Russian infighting could further hamper Moscow’s war effort as the early stages of a Ukrainian counter-offensive unfold.

Conservative chairman of the Commons Defence Committee Tobias Ellwood described the developments as a “huge opportunity for Ukraine to exploit the current mutiny and chaos in Russia”.

Tory former Cabinet minister Simon Clarke tweeted: “Incredibly serious, and a day when we should remember with thanks our exceptional intelligence and military community who will be monitoring this closely.”