Special Forces servicemen have been 'reminded' of the rules after an image was shared of troops in an unkept state.
Soldiers were seen sporting shagging beards and long hair in the snap - with two even wearing a baseball cap.
The photo, taken during training in Kent, reportedly provoked an angry response by senior commanders.
Members of the SAS and the Special Forces Support Group were a police station in Maidstone following an exercise.
The wearing of caps with military uniform is a breach of the Army's dress code and they have been told they must abide with the King's Regulations when in Britain.
But it wasn't just the clothings that ticked off bosses.
Two of men also had hair 'longer than collar length' with a full beard also on show.
Regulations are that servicemen are to be cleanly shaven, with their hair cut over the ears.
It also states that "personnel should also not mix civilian and military clothing".
The Special Forces have always been given a certain amount of freedom when serving overseas.
But while based in barracks, strict dress code and behaviour are demands in the Armed Forces at all times.
"The Special Forces have been reminded that there is no need for them to retain beards and have long hair once they return to the UK," an Army source told The Mail on Sunday.
"As with any soldier serving in the British Army they are expected to present themselves correctly. An unkempt appearance is not acceptable irrespective of whichever regiment a soldier serves in."
A Ministry of Defence spokesman said the Government did not comment on the Special Forces.
When deployed overseas, Special Reconnaissance Regiment troops often dress to 'fit in' with the community in which they are based.
In Iraq and Afghanistan, they dressed as locals and were required to grow beards.
