Russia has accused Britain of being drawn "too deep" into the conflict in Ukraine, warning that it risks a "dangerous" escalation of the crisis.
Andrey Kelin, the Russian ambassador to the UK, claimed British "specialists" had been involved in an audacious drone attack on Russia's Black Sea fleet at the weekend in which three warships were reportedly damaged, including the flagship HMS Makarov.
After Britain's ambassador to Moscow, Deborah Bronnert, was summoned to the Russian foreign ministry in connection with the claims, Mr Kelin said the Kremlin would publish its proof "pretty soon".
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"We perfectly know about the participation of British specialists in training, preparation and execution of plans against the Russian infrastructure and the Russian fleet in the Black Sea. We know that it has been done," he said.
Watch: The moment Ukrainian drones carrying explosives are believed to attack Russian Black Sea fleet.
"It is dangerous. It can bring us to the line of I would say no return – return is always possible – but we should avoid escalation.
"This is a warning actually that Britain is too deep in this conflict. It means that the situation is becoming more and more dangerous," Mr Kelin added.
Mr Kelin said Russia was also continuing to investigate the explosions which damaged the Nord Stream 1 and 2 undersea gas pipelines in September, adding that UK special forces had been involved in training the Ukrainians in the use of explosives and undersea drones.
In the West, it is widely suspected that the blasts were the work of the Russians themselves in an attempt to increase the pressure on European countries from rising energy prices.
A UK Government spokesman said the Russian allegations were an attempt to "distract attention" from its illegal invasion of Ukraine and its continuing losses on the battlefield.
"We do not plan to give a running commentary on these allegations; it is no secret that the United Kingdom has taken a public lead in our support to Ukraine – this has been enduring since Russia's illegal annexation of Crimea in 2014," the spokesman added.