6. c. 21) was an Act of the Parliament of the United Kingdom that allowed certain British Allies during the Second World War to set up maritime courts with criminal jurisdiction within the United Kingdom.

[2][3] The Act came into need because of the early events of the Second World War, in which the remnants of the European anti-Nazi forces fled to Britain after their defeat.

Finding their armed forces in Britain, sometimes with a large number of merchant navy ships, they had no effective machinery of justice.

[4]: 118 Section 1 of the Act allowed for new maritime courts to exercise jurisdiction over offences committed by any non-British person on a merchant vessel owned by the nation or power which constituted the court.

Section 2 allowed for the courts to hear cases against their own citizens involving mercantile conscription laws.