Britain's First Sea Lord has described the Royal Navy as a "necessity" and "not a luxury".
Admiral Sir Mark Stanhope, head of the Navy, argued for continued investment in the fleet as he spoke to the Royal United Services Institute in London.
His comments come as the government conducts a major defence review which will consider the size of the fleet and the future of Portsmouth Naval Base.
It is examining what sort of armed forces are needed and cost involved.
The review is the first to take place since 1997 and defence secretary Liam Fox has refused to rule out a cut in troop numbers.
Adm Stanhope said: "Maritime capabilities are not a luxury, they are a necessity.
"Our ability to control what happens at sea and from the sea is fundamental to our national security and our prosperity."