Sailors from the Royal Navy are to perform the Changing the Guard outside Buckingham Palace for the first time in the ceremony's 357-year history.
Eighty-six sailors from 45 Royal Navy ships and establishments have spent a month preparing ahead of the first ceremony on Sunday morning.
The ceremony is usually carried out by one of the five Foot Guards Regiments from the Army's Household Division.
The change in personnel is to mark 2017 as the "Year of the Navy".
Changing the Guard can be traced back to King Henry VII, when a royal bodyguard was first created.
It is the ceremony where one set of guards - the Old Guard - hands over the responsibility of protecting Buckingham Palace and St James's Palace to another set of guards - the New Guard.
The Old Guard march from the forecourt of Buckingham Palace at 11:00 GMT, accompanied by a regimental band.
The New Guard then march, again with a band, from Wellington Barracks to take over.
The ceremony itself has been taking place since the restoration of King Charles II in 1660.
The sailors have been training at the Royal Navy's headquarters in Portsmouth.
Lt Cdr Steve Elliott, 44, from Portsmouth, will be Captain of the Queen's Guard - believed to be the first in the Royal Navy since Sir Walter Raleigh in 1587.
The sailors will also be performing Royal Duties at St James's Palace, Windsor Castle and the Tower of London.