The Royal Yacht Britannia's Royal Barge and two escort boats have set off on their 400 mile road journey to London from Edinburgh.
The barge will be at the head of the royal procession of about 1,000 boats for the Thames Diamond Jubilee pageant on Sunday.
The vessel will carry the Queen and the Duke of Edinburgh to the pageant barge The Spirit of Chartwell.
The "Yotties", the Queen's one-time Royal Yachtsmen, will crew the boats.
The barge was used to transport the Queen to and from the Royal Yacht, until being decommissioned in 1997.
The coxswain at that time, Peter Young, will take the helm of the Royal Barge again on Sunday.
Britannia carried the Royal family on 968 official voyages.
It is said to have travelled over one million miles around the world.
In January, the yacht was taken into a dry dock for maintenance.
It was the first time the vessel had been moved in 14 years. About 2,500 litres of paint were used in the restoration work.