Experts are calling it the most important piece of Admiral Lord Nelson memorabilia to go under the hammer in the last century...
So intensely proud was Nelson of his medals that he ordered cloth and wire versions to be sewn to his uniform and worn during battle.
And the most important of those medals, in the eyes of experts, was the Breast Star of the Order of the Bath - a silver, gold and enamel star which was presented to Nelson in 1797.
The medal commemorates Nelson's exceptional victory at the 1797 Battle of Cape St Vincent in the Caribbean.
During the battle, Nelson disobeyed orders and broke his ship, HMS Captain away from Admiral Sir John Jervis's fleet and headed to engage the Spanish van.
Assisted by HMS Culloden, Captain took on all three ships - the 112-gun San Josef, the 80-gun San Nicolas and the 130-gun Santísima Trinidad. The confrontation resulted in heavy damage to Captain and Culloden.
Nevertheless, Nelson found himself alongside the San Nicolas. He led a boarding party across, crying "Westminster Abbey!" or "Glorious victory!" and successfully forced her surrender.
An artist's impression of Admiral Lord Nelson's capture of a Spanish
San Josef attempted to come to the San Nicolas's aid, but became entangled with the latter and rendered immobile. Nelson led his party onto the San Josef and captured her as well.
Although Nelson disobeyed orders, Jervis, who liked the Admiral, neither officially reprimanded Nelson nor mentioned his actions in his official report of the battle.
He "contributed very much to the fortune of the day", Jervis would later write in personal correspondence to the 2nd Earl Spencer.
After Nelson was later shot dead on the deck of HMS Victory during the 1805 Battle of Trafalgar, the medal was passed to his brother, the Reverend William Nelson.
William Nelson passed it to his brother's confident, Admiral Sir Richard Goodwin Keats - and it has remained in the Keats family ever since.
The Breast Star of the Order of the Bath will be auctioned at Sotheby's in London on October 22, which happens to be the day after Trafalgar Day.
It will carry an estimate of £300,000-500,000.
Many of Nelson's personal belongings and medals were stolen from an exhibition in Greenwich in 1900 which contributes to this piece's considerable value.