Since this date in 1864, the White Ensign has been for the exclusive use of Royal Naval ships and shore establishments, the Royal Yacht Squadron, Trinity House vessels escorting the Sovereign and certain other ships in preservation or in special service. HMS Belfast and HMS Cavalier are two such 'preserved' ships, and the ensign is kept 'flying' on the wrecks of HMS Royal Oak, HMS Prince of Wales and HMS Repulse, all three officially 'war graves'.
The Blue Ensign is now reserved for ships commanded by Royal Naval Reserve officers and certain Government Officers when it is 'defaced' by the addition of a badge showing the office. It was often worn by ships bearing the letters "RMS" (Royal Mail Steamer/Ship - denoting that ship was contracted to carry mails for the Royal Mail) in front of their names, though not because they carried the mails, but because their officers were members of the RN Reserve. Thus such famous ships as the old Cunard liners, RMS Queen Mary or Queen Elizabeth, and many of the Union Castle ships on the South African run wore the Blue ensign and carried the "RMS" designation.
The Red Ensign is reserved for merchant vessels, 'private' ships, and yachts registered in British ports. As with the Blue Ensign, it is sometimes seen being flown by government establishments or officers, and is then 'defaced' with a badge or crest denoting the agency or office.