A handwritten letter in which King George III signals his intent to go to war with France and Napoleon is to be sold at auction.

The document is dated May 14 1803 – four days before Britain formally declared war on France.

It was written by King George III to Lord Hawkesbury, then Secretary of State, and is described as a “defining moment of history” by auctioneers at Cheffins in Cambridge.

The letter from King George III
The letter’s pre-sale estimate is £500 to £1,000 (Cheffins/ PA)

In the letter, to be auctioned on January 10, the King wrote that he had “perused the dispatch and private letter from Lord Whitworth” – a British politician and diplomat who at the time of the letter was ambassador in Paris.

“The conduct of France has been equally unfair to the last,” he wrote. “And though conscious of the Evils that must be entailed on many Countries by the renewal of War, yet the conviction that by the restless disposition of the Ruler of France this event could not long have been kept off, it seems necessary to attend alone to the best modes of repelling the violence with effect, and the attacking those objects which our present means render attainable.

“The King will remain in Town to execute any Steps that the present moment may require.”

He signs off the letter as “George R”.

The letter, being sold by an anonymous private vendor, has a pre-sale estimate of £500 to £1,000.

Charles Ashton, a director at Cheffins said: “This letter is a defining moment of history showing the King’s intention to go to war with France and Napoleon.

“The Napoleonic wars are iconic of the huge victories of the British army and these were defining moments in our country’s history, laying the groundwork for the nation to rule the waves and put us at the forefront of European history.

“Whilst relations between Britain and France had been somewhat strained since the Treaty of Amiens in March 1802, this letter marks the end of peacetime negotiations and contains the explicit instructions from the King to proceed with war.”

The ensuing wars ended with the Battle of Waterloo in 1815.

Mr Ashton said letters fully written by George III are rarely seen at auction.

“This one gives us an insight into what was going through his mind at the time,” he said. “This could easily be bought by a collector or possibly find its way into an institutional collection or a museum.

“It really is a museum-worthy piece and could well end up in the royal archives.”