UK pirate radio (unlicensed illegal broadcasting) was popular in the 1960s and experienced another surge of interest in the 1980s. There are currently an estimated 150 pirate radio stations in the UK. A large proportion of these pirate radio stations operate in London, with significant clusters in Harlesden, Stoke Newington, Southwark and Lambeth.
"Pirate radio" in the UK first became widespread in the early 1960s when pop music stations such as Radio Caroline and Radio London started to broadcast on medium wave to the UK from offshore ships or disused sea forts. At the time these stations were not illegal because they were broadcasting from international waters. The stations were set up by entrepreneurs and music enthusiasts to meet the growing demand for pop and rock music, which was not catered for by the legal BBC Radio services.
The first British pirate radio station was Radio Caroline, which started broadcasting from a ship off the Essex coast in 1964. By 1967 twenty-one pirate radio stations were broadcasting to an estimated daily audience of 10 to 15 million. The format of this wave of pirate radio was influenced by Radio Luxembourg and American radio stations. Many followed a top 40 format with casual DJs, making UK pirate radio the antithesis of BBC radio at the time. Spurred on by the offshore stations, several landbased pirate stations took to the air on medium wave at weekends, such as Telstar 1 in 1965, and RFL in 1968.
According to Andrew Crisell UK pirate radio broke the BBC's virtual monopoly of radio to meet demand that had been neglected. In reaction to the popularity of pirate radio BBC radio was restructured in 1967, establishing BBC Radio 1, Radio 2, Radio 3 and Radio 4. A number of DJs of the newly created pop music service BBC Radio 1 came from pirate stations. The UK Government also closed the international waters loophole via the Marine Broadcasting Offences Act of 1967, although Radio Caroline continued to broadcast until March 1968.