To facilitate the flow of international trade, and to help the balance of payments of the countries operating them, a number of free-ports have been ; established throughout the world and six have now been established within the U.K. The advantage of a free-port is that goods can be moved through the area of the port without being subject to any customs or tariff charges or barriers. This is particularly significant in the context of entrepót trade. The absence of customs barriers does not permit the landing of cargoes that are prohibited by the country operating the port, but the privileges afforded and the tendency to relax supervision are not infrequently abused by the more unscrupulous traders.
Reference: The Penguin Business Dictionary, 3rd edt.