The Queen has conferred the title and office of Lord High Admiral of the Navy on HRH Duke of Edinburgh on the occasion of his 90th birthday.
Earlier the duke told the BBC he was "winding down" and reducing his workload as a senior royal.
In a BBC interview the Queen's husband said: "I reckon I've done my bit."
The Queen, who has been titular head of the navy since 1964, has given the title to the duke, who gave up his naval career to support her.
Prince Philip joined the navy in 1939 and was the best cadet at the Royal Naval College in Dartmouth that year. But he quit in 1951 after marrying the then Princess Elizabeth.
On Friday Prince Philip hosted an event for the Royal National Institute for Deaf People at Buckingham Palace, during which he was given a set of ear defenders as a birthday present. Receiving the gift, he joked: "Can you get Radio 3 on this?"
Ironically the charity used the event to drop its royal title and rebrand itself as Action on Hearing Loss.
During the event, which marked the charity's 100th birthday, the guests, who included former MP Ann Widdecombe and historian David Starkey, spontaneously began singing Happy Birthday.
He was also given a birthday card designed by 12 of the charity's service users who are deaf, or deaf and blind, and attend the organisation's workshop in Bath. It was added to the 2,000 other cards he received.
The duke, whose mother was born deaf, has been the charity's patron since 1958.
He said: "What I just want to do is to welcome you all here on this 100th birthday party - 100th you'll notice, not 90th - and say what a great pleasure it is to see you all here and to congratulate you all who have been involved in the success of this remarkable organisation."
Outside Buckingham Palace, the band of the Irish Guards played Happy Birthday for a crowd of tourists.
The Duke will attend a private service of thanksgiving at Windsor on Sunday.
His birthday has also been marked by a 62-gun salute by the Honourable Artillery Company.
The Royal Mint marked the milestone by producing a commemorative £5 coin, available in cupro-nickel, gold and silver, as well as a rare platinum edition at £5,450.
The souvenir piece features a portrait of Prince Philip on one side and the Queen on the other - the first time a reigning monarch and consort have appeared on opposite sides of a UK coin.
Sculptor Mark Richards, who designed the coin, said: "The challenge for me in creating this design was to capture a man who gives great support to the monarch and the country, while remaining largely in the background.
"Therefore I have focused on a close-up of his face with all its accumulated dignity, wisdom and experience."
The Queen turned 85 in April and will reach her Diamond Jubilee next year, marking 60 years since she came to the throne.
She married Prince Philip in 1947, making him the longest-serving consort in British history.
In the interview with the BBC, he talked about his advancing years, saying it was better to get out "before you reach your sell-by date".
"I reckon I've done my bit so I want to enjoy myself a bit now, with less responsibility, less frantic rushing about, less preparation, less trying to think of something to say," he said.
"On top of that, your memory's going - I can't remember names and things."
Over many decades, the duke has embraced a range of causes, including the Duke of Edinburgh's Award Scheme and conservation, though he insisted he was not a "green" campaigner.
"There's a difference between being concerned for the conservation of nature and being a bunny hugger," he added.
BBC royal correspondent Peter Hunt said the prince's comments were typically frank admissions.
He said the Duke of Edinburgh was a no-nonsense royal whose crucial role had been to support the Queen, especially during years of turmoil.
On the eve of his birthday Prince Philip, dressed in his Grenadier Guards uniform, took the salute at the annual beating retreat ceremony on Horse Guards Parade.
Watched by 4,000 members of the public, almost 300 members of bands of the Household Division performed under a blue evening sky.
The division was supported by the King's Troop Royal Horse Artillery and the United States Army Europe Band and Chorus.
The latter brought a touch of wartime nostalgia to the ceremony, singing the White Cliffs Of Dover and slow-dancing in pairs.
The origins of the beating retreat ceremony lie in the early days of chivalry when beating or sounding retreat called a halt to the day's fighting, a return to camp and the mounting of the guard for the night.