Britain’s armed forces chief has dismissed speculation that Russian President Vladimir Putin is “not well” or could be assassinated as “wishful thinking”.

The UK Chief of the Defence Staff Admiral Sir Tony Radakin was asked if President Putin, who launched an invasion of Ukraine in February, could be “toppled” or face “regime change”.

But he told Sophie Raworth on BBC One’s Sunday Morning show: “I think some of the comments that he’s not well or that actually surely somebody’s going to assassinate him or take him out, I think they’re wishful thinking.

“As military professionals we see a relatively stable regime in Russia, President Putin has been able to quash any opposition, we see a hierarchy that is invested in President Putin and so nobody at the top has got the motivation to challenge President Putin, and that is bleak.”

He said the “challenge of Russia is going to endure” potentially for “decades in terms of Russia as a threat”, and the next prime minister should know that Russia is “the biggest threat” to the UK.

He added: “So the biggest threat is Russia, and that’s Russia in all its guises when you look at it militarily.

“Its land forces are probably less of a threat in the short term because of that degradation, that depletion that we’re seeing with their struggle in Ukraine.

“But Russia continues to be a nuclear power, it’s got cyber capabilities, it’s got space capabilities, and it’s got particular programmes under water so it can threaten the underwater cables that allow the world’s information to transit around the whole globe.”

On what Britain’s armed forces would look like if defence spending increased to 3% of GDP, he said: “I think they would be even more modern and they would have even more punch and they would have even more impact around the world, and that’s clearly a choice for government.”

Reflecting on a recent visit to Kyiv, he said Ukraine’s army “absolutely” believed it will win the war.

He added: “They plan to restore the whole of their territory in terms of Ukraine, and they see a Russia that is struggling, a Russia that we assess has lost more than 30% of its land combat effectiveness.

“What that actually means is 50,000 Russian soldiers that have either died or been injured in this conflict, nearly 1,700 Russian tanks destroyed, nearly 4,000 armoured fighting vehicles that belong to Russia destroyed.”

On how he will brief the next Prime Minister, he added: “We will always give them a brief on the current situation so they are aware of where we have our armed forces.

“That’s dominated by Ukraine and the support that we are providing to Ukraine but we also try to step back and give a wider threat picture.

“And then we have to remind the Prime Minister of the extraordinary responsibility they have with the UK as a nuclear power, and that is part of the initiation for a new British Prime Minister.”