Admiral Nelson is believed to have used the historic weapon in battles against the French during the Napoleonic Wars.
He gave the blade to his nephew William Maurice Suckling – who served under him on HMS Agamemnon – before his death at the Battle of Trafalgar in 1805.
An account of the 24-inch sword's history written by the famed naval hero's great-nephew Horatio, the third Earl Nelson, included the remarkable claim that the sword also once belonged to Bonnie Prince Charlie, who laid claim to the British throne in the 18th century.
In the document – unearthed by maritime antiques dealer Charles Wallrock, who is selling the blade – Earl Nelson claims the sword was gifted to his famous ancestor by Cardinal Henry York, who said his brother Bonnie Prince Charlie had used it all his life.
And while the claim cannot be corroborated and the story is on the face of it incompatible with the known dates, it remains a curious account without explanation.
What is certain is that Nelson gave the sword to his relative William Maurice Suckling, whose family still possessed it when the third Earl wrote his account.
It then passed down the family and was ultimately bought by Charles Wallrock who said: ‘There is no doubt that this was Nelson’s sword – he may well have used it in action against the French.
‘The curved steel blade is just short of 24 inches and dates from the 17th century. It is mounted into a later silver hilt, hallmarked "Nixon”, London 1752.
‘That the fairly ordinary blade was remounted in silver by Nixon certainly suggests it was deemed important.
‘It retains its leather scabbard and has now been mounted in a display cabinet befitting its importance.
‘The third Earl’s account was based on a previous history from 1823, but Maurice Suckling had died three years earlier so was never able to state whether there was any truth in it.
‘The account states that Bonnie Prince Charlie’s brother – Cardinal York - was in Italy on the run from Napoleon.
‘Nelson was cruising off the coast in 1796 and decided to help him, even though he was a Catholic.
‘The last of the Stuarts was in rags, so Nelson invited him on board, looked after him then set him ashore with money to defray his expenses.
‘When some time later they met again in Genoa, Cardinal York gave him the sword as a thank you for his generosity. He told him it had belonged to his younger brother.
‘The account – or a version of it – might be true but I have been unable to corroborate it. There are other suggestions about how Nelson acquired this sword.
‘However, it adds another layer of intrigue on this historic artefact that was owned by one of the greatest naval heroes of all time.’
The sword will be on display at the Chelsea Antiques Fair later this month.