British Act of Parliament aimed at restricting the trade between the Isle of Man and Britain and Ireland before Revestment.

George Grenville introduced the bill in the House of Commons on 21 January 1765 under the title “An Act for more effectually preventing the mischiefs arising to the revenue and commerce of Great Britain and Ireland from the illicit and clandestine trade to and from the Isle of Man”. The Manx Society account identifies it as 5 Geo. III c.39 and notes that it was commonly called the Mischief Act.

The Act authorised British customs and excise officers to search ships within the Island and to make seizures by land or water. It also allowed prosecutions relating to seizures and offenders to proceed in Britain, Ireland or in courts on the Island held in the King’s name, with British or Irish legal process capable of being served in Mann.

The legislation was directed against the running trade, but the later Manx commentary objected to the Act’s description of all trade to and from Mann as illicit. Goods could be lawfully imported into and exported from the Island under Manx law; illegality arose when those goods were landed in Britain or Ireland in breach of their customs laws.

The Duke of Atholl petitioned the Commons against the bill on 13 February 1765 and was heard by counsel. The measure continued through Parliament while negotiations for purchase of his rights were revived.