The Army is investigating after a soldier was spotted wearing earphones while on duty outside Horse Guards in central London.
The Household Cavalry Mounted Regiment Trooper was pictured on Monday morning apparently listening to music using white Apple earphones, with a sword in his hand.
An Army spokesman said: "We are currently investigating images of a soldier on duty at Horse Guards today wearing an earpiece.
"If a member of the Armed Forces is found to have fallen below the high standards expected, appropriate action will be taken".
An observer told MailOnline: "He's got 18th century armour on and a 2017 headset. It's all a bit odd."
The Household Cavalry Mounted Regiment dates back to 1660. As well as playing a ceremonial role, the soldiers are also involved in combat operations.
The soldiers perform roles including the changing of the guard on Horse Guards Parade, taking part in the State Opening of Parliament and formal events on Remembrance Sunday.