More than 20 Royal Navy vessels are to gather in Plymouth to mark the Queen's presentation of a new Colour to the fleet.
The Queen will be on board HMS Ocean for the ceremony
A crowd of about 20,000 is expected to enjoy Wednesday's spectacle - which will be the biggest collection of Royal Navy ships and submarines the city has seen since 1905.
It is the first ceremony of its kind for more than 30 years and only the third in the navy's history.
The presentation will be held on board the Royal Navy's largest warship, Plymouth-based helicopter carrier HMS Ocean.
More than 20 aircraft from RNAS Culdrose in west Cornwall and RNAS Yeovilton in Somerset will join a formation flypast.
The Queen will then have lunch with 40 of the navy's most senior officers and meet families of naval personnel at the Devonport naval base.
In the afternoon, she will meet more navy families and the public on The Hoe.
The Queen will also name a new lifeboat at Queen Anne's Battery.
Plymouth Police, Royal Navy and the Ministry of Defence police are joining forces amid tight security for the event.
Ministry of Defence police will escort the royal party from Mayflower Steps to HMS Ocean.
The assault ship, along with others, will be guarded by police launches and inflatable boats.
Many city schoolchildren have been given the day off for the event.
The first Fleet Colour was presented by King George V in 1926 and the second by the Queen in 1969.
But this is the biggest gathering of the fleet in Plymouth Sound since 1905, when 35 warships gathered for the commemoration of King Edward VII's birthday.
The Queen has approved the navy's use of the Colour - known as the Queen's Colour - which is a double-folded silk White Ensign.
The flag bears a crown and Royal Cypher, with gold and blue silk cord and gold tassels hanging from the ensign staff.
Unlike the Colours of the Army, which carry battle honours and vary in design from regiment to regiment, the Queens' Colours of the Royal Navy do not alter from command to command.
The old ensign will be placed in St Nicholas Church at HMS Drake in Plymouth.