Britain needs two aircraft carriers, not one, if it is to provide a credible answer to the global threat to maritime security, the head of the Royal Navy has said.
Admiral Sir George Zambellas, who is First Sea Lord and chief of the naval staff, said having two carriers ensuring continuous availability was "a modest extra premium to pay" for an "effective, credible, available, insurance policy".
His comments come just days before the navy's new aircraft carrier and biggest ever ship, the HMS Queen Elizabeth, is formally named by the Queen.
Admiral Zambellas was delivering the keynote speech at the Rusi (Royal United Services Institutes) International Sea Power Conference 2014 in Whitehall.
He said that the Queen Elizabeth class ships would deliver more than carrier strike for the British navy, but highlighted the importance of having two - not one.
"Credibility also hinges on a carrier being available when the need arises," he said. " Hope is not a reliable method of ensuring capability availability when a crisis erupts.
"That is why we need the effects of a UK carrier - it's the wrong moment to find out that nothing happens when you push the carrier button.
"So to ensure continuous carrier availability that means having two carriers, not one - a decision for government in next year's SDSR (Strategic Defence and Security Review) of course, but this is a modest extra premium to pay, for an effective, a credible, an available, insurance policy."
Admiral Zambellas' comments echo those made by former head of the Royal Navy Admiral Lord West of Spithead, who last month said that without an increase in defence spending, the nation was "on the road to disaster" and "balanced on a knife edge".
Lord West criticised plans for the near-completed second aircraft carrier HMS Prince of Wales, which suggested tying the latter up or selling it for a "bargain basement price".
He said: "This means that instead of our nation having a carrier available 100% of the time - and my goodness me in the next 50 years I promise you our country will need it, sadly we will - we will only have one available 80% of the time and of course in a national emergency we could have had two carriers.
"All my experience and I'm sure many of yours tells you when a national crisis arises it will be in that 20% of downtime. That is the way it goes."
HMS Queen Elizabeth will be formally named by the Queen on Friday, when she will smash a bottle of whisky against it at Rosyth in Fife, where the 65,000-tonne aircraft carrier has been assembled and fitted out.
The ship and a second vessel, the under-construction HMS Prince of Wales, are both termed Queen Elizabeth Class aircraft carriers and are the largest warships ever built for the Royal Navy. Assembly of HMS Prince of Wales is set to begin at Rosyth later this year.
Those behind the project, which costs an estimated £6.2 billion overall, say the QE Class will be the centrepiece of Britain's naval capability.
Speaking at Rusi today, Admiral Zambellas said the same principle of "insurance" of having two carriers applied to the UK's nuclear deterrent.
He said: " A posture which delivers less than 100% availability is not available, and therefore not credible - not just in the eyes of potential adversaries, but in the eyes of our key strategic partners as well.
"That is why the current UK Government supports a continuous at-sea deterrent, a position supported by our Opposition defence spokesman just last week.
"In my professional judgment, that means four boats in the new Successor class - to replace our four current V boats.
"Credibility also means investment in ships, submarines and aircraft that are capable of credible standards of war-fighting.
"In our case, the Queen Elizabeth class carriers, Astute class submarines, the Type 45, the Type 26, Protected Mobility for the Royal Marines and Lightning II, plus the tankers.
"Because, to put it bluntly, in our line of business, there are no prizes for coming second. They all contribute to our strategic authority.
"As our CDS (Chief of the Defence Staff) has said, here at Rusi, ' If the United Kingdom wants to stay in the Premier League of smart power then it must invest in Armed Forces that can generate hard power capability that is credible in respect of conventional coercion and deterrence'."
Admiral Zambellas said t h e HMS Queen Elizabeth was "a clear statement of the UK's strategic maritime ambition" and a "return to the scale, professional complexity and responsibility of the carrier strike capability last operated by the Royal Navy and Royal Air Force four decades ago".
Amid global threats, Britain's maritime forces must be sufficiently forward deployed, at high enough readiness, and enough scale, he said, adding: " For us in the UK, there is no question of retreating to our island fortress.
"We do have our own shores to protect but our key role is out there in the world - helping to shape and influence international events - that is where you will find our maritime forces."
He said: " The world we live in is no less uncertain - as current events from Ukraine, Syria and Iraq daily testify.
"But - as over a decade of enduring land campaigns draws to a close - there is much less appetite to put boots on the ground."
He said "maritime power projection" could meet the challenge, responding rapidly to events across the globe and could give politicians and diplomats a "simple sea choice".
"Take, for example, our new aircraft carrier. In times of crisis and tension, it will offer a visible coercive presence or position sometimes out of sight, a flexible means of escalating and de-escalating as the national or international will dictates.
"So, maritime power projection offers utility. Not just military utility, of course, but political utility - hard and soft power delivered from the sea. Less boots on the ground - more boots from the sea."
He said in the coming decades the sea would become more, not less, important, with security challenges coming from climate change, with new obligations in the High North, as well as pressure on marine resources due to an expanding global population.