Admiral Sir Ben Key attributed the challenge to a lack of debate about what it means for the UK to be a nuclear-armed power - a fundamental pillar of its security.

"I think it is fair [to say] that this country is not very good about talking about […] nuclear power as opposed to nuclear weapons," the First Sea Lord told The House magazine.

While understanding why some people would be uncomfortable with the concept of nuclear power, he stressed that at sea it is "extraordinarily safe".

The Royal Navy's submarine service - also known as the silent service - operates four Vanguard-class, nuclear-armed ballistic missile submarines as well as the Astute-class nuclear-powered fleet, which is armed with conventional rather than nuclear warheads.

The nuclear-armed boats take it in turns to operate in secret for months out at sea.

Their core task is to ensure the UK always - 24 hours a day, seven days a week - has the ability to deploy a nuclear weapon against a target if needed.

This continuous at-sea deterrent - which has existed since 1969 - is designed to deter an enemy from launching nuclear weapons against the UK for fear of suffering the same fate: mutually assured destruction.

However, sustaining the deterrent requires a sufficient number of submariners who are willing to regularly spend months underwater without the ability to contact home - often without even knowing where in the world they are deploying.

'War for talent'

In an unusually frank admission about what is typically a top secret part of the navy, Admiral Key was quoted as saying that recruiting for the submarine service was proving difficult.

"I'm not going to sit there and say that we are awash with people," he told The House.

He revealed the navy is investing in outreach teams to explain to potential new recruits what life is like on a submarine.

"If you're thinking of joining a submarine service as a young person, you want to go and talk to a young submariner and find out what it's really like," he said.

More broadly, the admiral said his service was in a "war for talent" as the navy starts to regrow its workforce after decades of cost-cutting shrinkage.

"We are effectively in a war for talent in this country - there is no great secret in that," the First Sea Lord said.

"One of the challenges is actually, the navy of today, at 29,000 in a population of…about 65 million, actually, there are very few people who have got direct experience of coming from a naval family. Whereas if you track back 100 years, a lot of people had experience of a military family or a naval family."

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Recruits also expect more in terms of communication.

The admiral - who at the age of 57 has served in the navy for the past 39 years - recalled once returning from a six-month trip to be greeted by his wife and sons. One of them, who was two years old at the time, did not recognise him.

Now, "expectations of contact with people you love are changing [and] the ability for near-permanent connectivity cannot be met if you are in a submarine", he said.

The comments about submarine recruitment come as the navy seeks to expand its nuclear-powered submarine fleet as part of a new strategic partnership with Australia and the US - a move that will also require more submariners.