The report came from Russian state TV anchor Dmitry Kiselyov, who spoke as a simulation of an underwater missile being set off in the Atlantic Ocean with the aim of attacking Great Britain.
Despite there being no mention of Ireland in the broadcast, the clip shows the country being obliterated by the weapon - a giant nuclear-capable torpedo dubbed Poseidon - first.
Kiselyov, who is known as “Putin's mouthpiece”, explains: “Another option is to plunge Britain into the depths of sea using Russia's unmanned underwater vehicle Poisedon.
“It approaches its target at a depth of 1km at a speed of 200km/h. There's no way of stopping this underwater drone. The warhead on it has a yield of up to 100 megatonnes.
“The explosion of this thermonuclear torpedo by Britain's coastline will cause a gigantic tsunami wave up to 500m high.
“Such a barrage alone also carries extreme doses of radiation. Having passed over the British Isles, it will turn what might be left of them into a radioactive desert.”
Before this, Kiselyov said that Russia would first strike with its hypersonic missile known as Satan-2.
A clip of the news broadcast is being widely shared online, with many dismissing the video as “pure bonkers.”
One person wrote: “Just to quell everyone’s fears, no nuke could take out an entire country let alone two. City killers yes, once the aul westerly wind is blowing, we’ll be grand.”
Another said: “That animation is pure bonkers. A single nuke blast radius not only covering Britain and Ireland, but vaporising them.
“Surely there’s a (un) healthy dose of dark Russian humour here? If not, they’ve genuinely no idea what they’re on about.”
Some made light of the situation, with one man joking: “Ah now, they were fair accurate. Only UK and Northern Ireland were wiped out and maybe the lad who lashed his lorry into the embassy. Apart from that, we were absolutely fine.”
But others were concerned about the potential catastrophe posed by Russia’s weapons.
“Given what the Russians are doing in Ukraine, and what help we can expect if that were to happen, this is not news to be taken lightly, is it? Scary stuff,” one person wrote.