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The memo was written to Captain Courtney Boyle on the morning of January 19, 1805 and instructs Boyle, in his ship the Seahorse, to search for the French fleet round the southern end of Sardinia. The previous day, the French, led by Admiral Villeneuve, had managed to slip out of the port of Toulon, which Boyle and his fleet had been blockading for several months.

Nelson confirms the action outlined in the memo in his diary entry for January 19: “Sent Seahorse round the Southern end of Sardinia, to St Peter’s, to look out for them, but to prevent the Enemy, as much as possible, from seeing her; and the moment Captain Boyle discovered them, to return to me.”

The memo, which is being offered by an educational institution, has been authenticated by Victor Sharman, chairman of the Nelson Society, and Ernest Coleman, a local Nelson enthusiast and former Officer of the Day on HMS Victory.

Stephen Smallwood, the book and manuscript expert for Thomas Mawer & Son, believes the memo signals the first action Nelson took in the Trafalgar campaign. “Any Nelson letter is a rarity, but this memo set in motion a series of events which brought the French fleet to battle off Cape Trafalgar in 1805. This, I suggest, is an important chain in these events and may indeed be the first written record of any part of the Trafalgar campaign.”

As a unique item, the memo is almost impossible to value, but it will carry an estimate of £3000-5000.