Ukraine not Russia has the “strategic patience” to win the war after the Wagner Group revolt against Vladimir Putin, Britain said on Monday.
Foreign Secretary James Cleverly added that Kyiv’s forces were “steadily clawing back their territory” with the summer counter-offensive.
Updating the Commons after the aborted mutiny led by Wagner chief Yevgeny Prigozhin, Mr Cleverly said: “This weekend’s events show that it is Ukraine and its partners, not Russia, that have the strategic patience and resolve to prevail.
“Prigozhin’s rebellion is an unprecedented challenge to President Putin’s authority and it is clear cracks are emerging in Russian support for the war.”
He also argued that the scathing criticism of Putin’s case for war by the Wagner boss showed it was unjustified and unprovoked.
Prigozhin "drove a coach and horses through President Putin's case for war", Mr Cleverly told MPs.
He added: "Now that Russia's leadership cannot justify this war, even to each other, the only rightful course is for Putin to withdraw his troops and end this bloodshed now."
The Foreign Secretary explained in the Commons that Britain had planned for the prospect of instability erupting in Russia but stressed it did not want to interfere with the country’s internal political situation.
Earlier, ex-MI6 chief Sir Alex Younger said Putin’s authority has been “significantly weakened” by the Wagner Group rebellion.
The former head of Britain’s Secret Intelligence Service, said Yevgeny Prigozhin “impetuous” decision to march his forces into Rostov and then towards Moscow had little chance of succeeding.
But he added that the revolt had significantly undermined the Russian president.
He told BBC Radio 4’s Today programme: “For Putin’s part, he has a reputation as an implacable and decisive strongman.
“He’s actually a very indecisive character. He kicked this can (the clash between Prigozhin and his military chiefs) down the road.
“Then when it happened, on the one hand threatened blood-curdling revenge and then did a deal and offered clemency.
“So everyone comes out of this weaker.”
He rejected any idea that the West and “dark forces” were involved in the Wagner revolt, as was expected to be claimed by Putin propagandists, stressing it would not have wanted to promote “someone who does business with a sledgehammer,” referring to a particularly brutal execution of a Wagner soldier who defected to Ukraine before being recaptured.
He explained further: “The alternative...is that fundamentally Putin is losing his legitimacy, and that the deal that the Russian people have had to accept, all be it grudgingly, that they accept rule from his corrupt elite in return for some stability is eroding.
“He could hang around for a long time, but fundamentally I think his authority has been significantly weakened.”
Russia sought to restore calm on Monday after the aborted mutiny by Wagner Group mercenaries over the weekend, while Western allies assessed how Putin might seek to reassert authority and what it could mean for the war in Ukraine.
Ending their short-lived mutiny, Wagner fighters halted their rapid advance on Moscow, withdrew from the southern Russian city of Rostov and headed back to their bases late on Saturday under a deal that guaranteed their safety.
Their commander, Prigozhin, would reportedly move to Belarus under the deal mediated by Belarusian President Alexander Lukashenko, though he has not been seen since leaving Rostov in the back of a car.
Former soldier Sir Alex believes Prigozhin turned his forces back on the road to Moscow as he realised they were storming towards defeat.
“The key point is he wasn’t really prepared, he didn’t have a plan, he didn’t have enough people,” said the former spy who was known as C as MI6 chief.
“He achieved surprise in Rostov because he turned up unannounced.
“But I think it rapidly became clear, particularly when the shooting began, that he didn’t really have any prospect of succeeding.
“No-one was going to join him.”
Monday has been declared a non-working day in Moscow to allow time for things to settle, and there was little evidence of increased security in the capital on Sunday evening.
Defence Minister Sergei Shoigu, one of the main targets of Prigozhin’s anger, visited Russian troops involved in the military operation in Ukraine, state-run RIA news agency reported on Monday while providing no details on when and where.
But Putin, who has held power for more than two decades, had by Monday morning still to comment publicly since the de-escalation of one of the biggest challenges to his rule.
Russia’s General Staff chief General Valery Gerasimov - like Shoigu, a target of Prigozhin's ire - has also not been publicly seen or heard from since the rebellion.
Moscow Mayor Sergey Sobyanin announced an end to the "counter-terrorism regime" imposed on the capital Saturday, during which troops and armoured vehicles set up checkpoints on the edges of the city and authorities tore up roads leading into the city.
Confusion over the weekend’s extraordinary events has left governments, both friendly and hostile to Russia, groping for answers to what could happen next in the country with the world’s largest nuclear arsenal.
US Secretary of State Antony Blinken suggested the turmoil could take months to play out.
NATO Secretary-General Jens Stoltenberg said on Monday that the aborted mutiny by the Wagner mercenary group demonstrated that Putin had committed a strategic mistake by waging war on Ukraine.
"The events over the weekend are an internal Russian matter, and yet another demonstration of the big strategic mistake that President (Vladimir) Putin made with his illegal annexation of Crimea and the war against Ukraine," he said on a visit to Lithuania's capital Vilnius, ahead of a Nato summit there next month.
"As Russia continues its assault, it is even more important to continue our support to Ukraine."
EU foreign policy chief Josep Borrell said that “the monster that Putin created with Wagner, the monster is biting him now”.
Speaking ahead of a meeting of foreign ministers in Luxembourg, he added on Russia’s internal conflict: "The political system is showing fragilities, and the military power is cracking.
“The monster that (Russian President Vladimir) Putin created with Wagner, the monster is biting him now, the monster is acting against his creator.”
But he also stressed: "It's not a good thing to see that a nuclear power like Russia can go into a phase of political instability," adding this was the moment for the EU to continue supporting Ukraine more than ever.
