The Immigration (Ports of Entry) Order 1987
Under paragraph 26 of Schedule 2 to the Immigration Act 1971 the owners or agents of a ship or aircraft employed to carry passengers for reward are prohibited from arranging, without the approval of the Secretary of State, for the ship or aircraft to call at a port in the United Kingdom other than a designated port of entry for the purpose of disembarking passengers, if any of them require leave to enter the United Kingdom and have not been given leave, or for the purpose of embarking passengers unless the owners or agents have reasonable cause to believe all of them to be British citizens. An owner or agent who contravenes the provisions of paragraph 26 commits an offence under section 27 of the Act.