http://www.legislation.gov.uk/id/ukpga/Vict/33-34/90

http://www.legislation.gov.uk/ukpga/Vict/33-34/90/section/31

Foreign Enlistment Act 1870

An Act to regulate the conduct of Her Majesty’s Subjects during the Existence of Hostilities between Foreign States with which Her Majesty is at peace.