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British Royal Flags: Prince Philip, the Duke of Edinburgh

image by Graham Bartram

Source: World Flag Database

Standard
Duke's flag, before Elizabeth's ascension
Philip Mountbatten (before his marriage)
Racing Flag Private Signal

See also:

Standard

Prince Philip was consort to Queen Elizabeth. His standard was quartered Denmark, Greece, Mountbatten, and Edinburgh: the two royal families the prince descends from, his surname, and his title. Denmark: three blue lions passant on a yellow field strewn with nine sea-leaves, depicted as red hearts. Greece: a white centered cross on a blue field. Mountbatten: 5 vertical stripes of white and black. Edinburgh: on a white field, a black three-towered castle, with white details except for red roof, banners, and gate, on a rocky grey base with a stairway leading up to the gate (usually 1:2, rarely 2:3).
Peter Hans van den Muijzenberg, 24 April 2002

Prince Philip adopted the surname Mountbatten prior to his marriage as a compromise because of the difficulties attached to his own ancestral surname. For one thing, the Houses of Denmark and of Greece did not use a surname. For another, the surname they were entitled to use was Oldenburg. However, following the two World Wars there was considerable anti-German feeling in Britain, so that also was not deemed suitable. Its English translation, Oldcastle, recalled a comic character from Shakespeare.
His personal flag is a banner of arms, and was flown when he was present aboard ship. I am not certain what rules related to its use on land, since it would possibly have been superseded by the royal banner (Royal Standard) when the Queen was present.
Mike Oettle, 30 May 2007

A drawing in "Simple Heraldry (subtitle Cheerfully Illustrated)" by Iain Moncreiffe and Don Pottinger (Thomas Nelson and Sons) shows the derivation of the Duke of Edinburgh's coat of arms, with it whimsically used as a longboat sail. The reference mentions instances where the arms of places derive from people, and this is an instance of where the arms of a place became part of a personal coat of arms - so you have Edinburgh's arms showing a castle, and that same castle as the fourth quarter in Philip's arms.
Mike Oettle, 25 September 2002

Duke's flag, before Elizabeth's ascension

image by Klaus-Michael Schneider, 20 October 2010

Flag of the Duke of Edinburgh (1951-1952)
According to Carr (1953) the design of the Duke’s standard, approved by His late Majesty King George VI in November 1951, consisted of the arms of the Duke impaled with those borne by Her Majesty Queen Elizabeth II, when she was the Heiress Presumptive. However since Queen Elizabeth came to the Throne the Duke has used the standard consisting of his arms only.
Source: Carr (1953), p.26
Klaus-Michael Schneider, 20 October 2010

Philip Mountbatten (before his marriage)

Before his marriage, Prince Philip bore arms as a prince of Greece (Greece with an inescutcheon of Denmark – the full royal Danish arms with all the quarterings and inescutcheons) – with no representation of Battenberg/Mountbatten at all, since that was his mother's family, and she was not an heiress. I do not know whether he had an armorial banner at that stage. Oldenburg appears in the arms of Denmark as an inescutcheon (in the earlier version of the Danish arms, the Oldenburg arms formed an inescutcheon on an inescutcheon). After his marriage he was (1947-52) husband of the Heir Presumptive, not initially consort.
Mike Oettle, 30 May 2007

Racing Flag Private Signal

image located by Peter Edwards, 18 March 2019

Racing Flag: Rectangle 4:5 (web image). Green field charged with a black stylized image of Edinburgh Castle on a white disk fimbriated black.
Source: accessed 1 March 2019, https://www.pinterest.ca/pin/151574343682736971/
"Bluebottle’s [Dragon Class] racing career got off to a credible start on 10 July 1948 when she finished third out of ten starters at the hands of Lt Cdr Crichton. Despite the absence of her Royal owners, Bluebottle flew the Royal couple’s racing flag which, for the first season, consisted of Princess Elizabeth and Prince Philip’s cypher in gold on a dark blue background. Prince Philip decided to replace this design during the winter of 1948/49 because he thought the detail was too complicated to see at a distance. Therefore, he opted for a dark green flag with a black heraldic Edinburgh Castle within a white circle."
Source: Johnstone-Bryden, Richard. The Royal Yacht Britannia: the official history. London: Conway Maritime Press, 2003.
Peter Edwards, 18 March 2019