Russia has threatened retaliation - including nuclear strikes - after British drones were used by Ukraine.
Moscow's embassy in London earlier accused the UK of escalating the Ukraine war, threatening consequences as it was confirmed two British suppliers manufactured drones were used to strike Russian industrial targets, including warehouses and refineries.
Chief Kremlin propogandist Vladimir Solovyov then declared that British arms factories and ships would be struck by Russia's underwater nuclear drone, named after the ancient Greek god of the seas.
"Hit them with Poseidon," he said in a broadcast on state TV.
"Give the British three days to learn to breathe underwater, then hit them with Poseidon."
Earlier, Moscow accused the UK of aiding "bloody crimes" by providing drones which struck logistics centres belonging to Wildberries, Russia's version of Amazon.
In a statement, the embassy said: "Reports that the Kyiv regime is using British drones to carry out strikes deep within Russian territory confirm that London is deliberately betting on an escalation of the Ukraine crisis, while hypocritically claiming to be striving for peace.
"In doing so, the United Kingdom is acting as an accomplice and co-perpetrator of the bloody crimes and terrorist attacks committed by Ukrainian neo-Nazis, seeking to contain Russia and inflict maximum damage on it by proxy.
"London's actions will inevitably carry consequences for which it will have to answer.
"The deeper its involvement in the conflict and the greater its support for Kyiv’s terrorist machinery, the higher the price it will pay."
In response, the Ministry of Defence (MoD) said the UK stood "shoulder to shoulder" with Ukraine.
"Russia should be in no doubt about the resolve of this Government to stand against Russian aggression, in Ukraine and against the UK and our allies," a spokesman said in a statement.
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Britain was "committed to providing the equipment Ukraine needs to defend itself against Putin's illegal invasion", the spokesman added.
Kyiv's strikes on Wildberries are estimated to have inflicted more £35billion worth of economic damage.
A military source confirmed to the BBC that British drones had been used in the strikes.
The threat follows a wave of Ukrainian strikes on Russia.
Governor of Moscow, Andrey Vorobyov, said a Wildberries warehouse had been struck in Tuesday's early hours.
And on Monday, nine people were killed and 23 injured, including a child, according to local officials.
Eight people were killed in the western Belogrod region, bordering Ukraine, and a woman was killed in the southern Astrakhan region.
On Saturday, Ukraine launched a strike described by Russia as the "largest-scale" this year.
That strike killed at least seven people, while a retaliatory strike from Russia on Ukraine killed at least seven and injured more than 39.
Russia launched its invasion of Ukraine in February 2022 and currently controls a fifth of Ukrainian territory.
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