Somersett case

…his judgment in the so-called Somersett case (1772), involving the slave James Somersett, who was bought in Virginia and attempted to run away after arriving in London, decided only that an escaping slave could not be forcibly removed from England for retributive punishment in a colony.

…took up, that of James Somersett (1772), that “as soon as any slave sets foot upon English territory, he becomes free.” (This decision did not include the colonies, however.)