A letter of marque was a commission authorising privately owned ships (known as privateers) to capture enemy merchant ships.

A letter of marque was issued by the High Court of Admiralty. Any captured vessels were then brought before admiralty courts for condemnation and sale.

Privateers were viewed as heroic and noble as opposed to pirates, who had no letters of marque, and were universally condemned as thieves and vagabonds.

The earliest letter of marque was issued in 1293 and they continued to be issued in times of war until privateering was abolished in 1856. 

Records relating to letters of marque and privateers are within the High Court of Admiralty records held by the National Archives

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