Wikipedia
The Marine, Etc., Broadcasting (Offences) Act 1967 c.41, shortened to Marine Broadcasting Offences Act, became law in the United Kingdom at 12 midnight on Monday, August 14, 1967. Its purpose was to extend the powers of the British Wireless Telegraphy Act of 1949 (which it incorporated by reference), beyond the territorial land mass and territorial waters of the UK to cover airspace and bodies of water.
At the time that the Bill was introduced in Parliament in 1966, there were radio stations and proposals for television stations outside British
licensing jurisdiction with signals aimed at Britain. These stations were at sea but there were press reports of stations planned from aircraft.
The Act included the Channel Islands and extended to the Isle of Man. As a result, offshore stations called pirate radio became criminal if operated or assisted by persons subject to UK law. Station operators thought they could continue if they were staffed, supplied and funded by non-British citizens, but this proved impractical.
More at http://en.wikipedia.org/wiki/Marine_Broadcasting_Offences_Act
There were reported to be 20 million listeners to the closedown of Radio
London on August 14 1967 at 15:00 UK time.
Radio Caroline continues – see http://www.radiocaroline.co.uk
The full story of offshore radio can be found in “Pop Went the Pirates II” by Keith Skues, see http://www.popwentthepirates.co.uk/
Info via DXLD