The next Prime Minister will be briefed on the war in Ukraine and reminded of the "extraordinary responsibility they have with the UK as a nuclear power", Admiral Sir Tony Radakin has said.
The UK Chief of the Defence Staff was asked by BBC One's Sunday Morning show what he will say to the new Prime Minister after they are elected by Conservative Party members to succeed Boris Johnson.
He said: "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 and that becomes the focus."
Admiral Radakin has also said the next Prime Minister should know that Russia is "the biggest threat" the UK is facing.
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He told the Sunday morning show: "So the biggest threat is Russia, and that's Russia in all its guises when you look at it militarily.
"So, 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 underwater so it can threaten the underwater cables that allow the world's information to transit around the whole globe.
"The challenge of Russia is going to endure way beyond 2022 and 2023 and 2024, this is going to go on for a long time… potentially decades in terms of Russia as a threat," he added.
Asked what Britain's Armed Forces would look like if defence spending increased to 3% of GDP, he said: "So, 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."