The Ministry of Defence is considering mothballing one of the giant £3 billion vessels after it is completed.
The call, by Admiral Sir George Zambellas, comes just days before the Queen is due to attend the official naming of the first ship, the HMS Queen Elizabeth, at Rosyth on Friday.
In a speech to the defence think-tank, Rusi (the Royal United Services Institutes) Admiral Zambellas said that Britain needed two aircraft carriers, not just one, to provide a credible answer to the global threat to maritime security.
He said the continuous availability that came with two carriers was "a modest extra premium to pay" for an "effective, credible, available insurance policy".
His comments echo similar remarks made by Defence Secretary Philip Hammond last year.
The Conservative cabinet minister called for the navy to have the use of both the Queen Elizabeth-class carriers. But Mr Hammond warned that while it would cost only £70 million a year to run an extra carrier, the money would have to be found from elsewhere in the defence budget.
A final decision on the future of the carriers is due to be made in the next Strategic Defence and Security Review, after the 2015 General Election.
Speaking during last year's Tory Party conference, Mr Hammond also described the cost of running the carriers as a "snip" compared to the amount spent building them.