Original battle plans drawn up by Admiral Horatio Nelson are to be auctioned by Bonhams in New York on 13 December.
The six page document was signed by the naval hero on the eve of the historic battle. It contains detailed instructions for his captains and hand-drawn illustrations of his tactics.
His revolutionary approach, called the 'Nelson touch', allowed his outnumbered fleet to defeat the French and Spanish navies on 21 October 1805 and thereby forced Napoleon to abandon his plans to invade England. It also established British naval supremacy for more than 100 years and made Nelson a national hero.
The battle plans and instructions were drawn up by Nelson on board his ship, the HMS Victory, at Cadiz, southern Spain, a little over a week before the engagement. The 'historically important' document has stunned specialists at Bonhams in New York where it is expected to spark an international bidding war next week.
Nelson outlined his strategy of splitting his fleet into three lines. He wrote: 'Something must be left to chance; nothing is sure in a Sea Fight beyond all others ... no Captain can do very wrong if he places his Ship alongside that of an Enemy.'
Valuer at Bonhams, Ian Ehling, said: "This is one of the most important documents in Naval history, and the purest expression of the 'Nelson Touch'. It shows Nelson's brilliance and, in particular, his notion of spontaneity in battle. He is giving his captains the freedom to act freely and decisively in response to the changing circumstances of the battle. That was an outstanding tactic, never seen before, which has been used ever since. He is recognising that some things must be left to chance - that is especially poignant when you consider that he would be shot and killed in the battle."
He was shot by a sniper onboard the Victory as the battle neared its end.