A sketch hand-drawn by Admiral Lord Nelson showing his plan for victory at Trafalgar has been discovered tucked inside the pages of a scrapbook after nearly 200 years.

The map was found by Martyn Downer, a historian who is an expert on Nelson, in a book dating from the 1830s which was recently sold at auction.

It shows his plan for splitting the Royal Navy fleet into three divisions to break and destroy the enemy French and Spanish lines coming out of Cadiz harbour.

Lines representing wind direction also appear on the attack plan, showing Nelson’s intention to attack across the wind to take advantage of increased speed.

The drawing, which has been donated to the National Museum of the Royal Navy in Portsmouth, was found alongside an address leaf bearing Nelson's signature and is dated September 5 1805.

Mr Downer said: "This fragment of pencil sketch is simultaneously a very intimate and vivid artefact, giving us an exceptional insight into Nelson's thinking and planning ahead of the battle.

"Its survival is quite remarkable, having been hidden away between the pages of a young lady's scrap book for the best part of 200 years."

Professor Dominic Tweddle, the museum's director general, said: "This is an astounding discovery, it shows that Nelson had thought deeply about how he would defeat the combined French and Spanish fleets long before he ever set foot on HMS Victory.