The 33-year-old was sporting a beard - something that is forbidden in the British Army, except in a few rare circumstances.
"He should have shaved it off for such an important day", a serving member of the cavalry regiment said in a Metro article.
According to reports in the United Kingdom media, the British Army only allows beards in certain circumstances, such as when shaving would conflict with a soldier's religion, when they are suffering from a skin complaint, or when they are special forces behind enemy lines.
As one Twitter user pointed out, King George V - Harry's great-great grandfather - had a beard when he placed the first wreath on the Cenotaph in 1919 while wearing a British Army uniform.
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The Queen and senior royals joined servicemen and women past and present at the annual Festival of Remembrance as the nation remembered its war dead on Armistice Day.
However, as he is no longer a serving member he is not required by the Ministry of Defence to comply with the Army's regulations.
According to Forces.net, the only military rank permitted to wear a beard while on official duty - during a parade, for instance - is the Pioneer Sergeant. Members of the Royal Family watched as Emeli Sande, Tom Odell and other stars performed alongside the Queen's Colour Squadron and The Band of HM Royal Marines.
The Queen, Harry's grandmother, is Colonel-in-Chief of the Blues and Royals, while the Colonel of the Regiment is Princess Anne, his aunt. "Five life sized bronze soldiers stand easy against a stone cenotaph, each representing one of the Guards' Regiments".
Britain's Prince Harry places a poppy as he attends the official opening ceremony of The Field of Remembrance at Westminster Abbey in London.