Russia’s naval commanders have moved submarines away from Crimea to safety after Ukrainian attacks near the peninsula.
They argued that the Russian president’s annexation of Crimea in 2014 was partly to ensure a base there for his Black Sea Fleet but this aim had now been weakened.
In its latest intelligence update, the Minstry of Defence in London said: “The command of Russia’s Black Sea Fleet has almost certainly relocated its KILO-class submarines from their home port of Sevastopol in Crimea to Novorossiysk in Krasnodar Krai, southern Russia.
“This is highly likely due to the recent change in the local security threat level in the face of increased Ukrainian long-range strike capability. In the last two months, the fleet headquarters and its main naval aviation airfield have been attacked.
“Guaranteeing the Black Sea Fleet’s Crimea basing was likely one of Russian President Vladimir Putin’s motivations for annexing the peninsula in 2014. Base security has now been directly undermined by Russia’s continued aggression against Ukraine.”
Britain, the US, Ukraine and other allies are fighting an information war against the Kremlin so their briefings need to be treated with caution, though, they are often far more believable than the propaganda being issued from Moscow.
In the latest update from the frontline, Ukraine said its troops had marched farther east into territory recently abandoned by Russia, paving the way for a potential assault on Moscow's occupation forces in the eastern industrial Donbas region as Kyiv seeks more Western arms.
Ukraine's armed forces regained control of the village of Bilohorivka and were preparing to retake all of Luhansk province from Russian occupiers, provincial Governor Serhiy Gaidai said.
The village is only 10 km (6 miles) west of Lysychansk city, which fell to the Russians after weeks of grinding battles in July.
“The occupiers are clearly in a panic,” said Ukraine’s president Volodymyr Zelensky in a televised address late on Monday, adding that he was now focused on “speed” in liberated areas.
He also stressed that investigators had discovered new evidence of torture used against some Ukrainian soldiers buried near Izyum, in the northeastern Kharkiv region.
The Kremlin rejected allegations that Russian forces had committed war crimes in Ukraine’s Kharkiv region as a “lie”.
Moscow regularly denies committing atrocities in the war or deliberately attacking civilians, though numerous reports, photographs and video footage from the frontline undermine this stance.
Russian forces struck the Pivdennoukrainsk nuclear power plant in Ukraine's southern Mykolaiv region but its reactors have not been damaged and are working normally, Ukraine's state nuclear company Energoatom said.
Meanwhile, Prime Minister Liz Truss said Britain next year will meet or exceed the £2.3 billion military aid spent on Ukraine in 2022.
UK military support is likely to include equipment such as multiple launch rocket systems, according to Government sources.
Germany will supply Ukraine with four more Panzer howitzer 2000 tanks along with an additional ammunition package, the defence ministry said. Ukraine has urged the West to step up military aid to help it turn the tide of the war against Russia.
Mr Zelensky has hinted he would use a video address to the United Nations General Assembly on Wednesday to call on countries to accelerate weapons and aid deliveries.