The MV Mi Amigo
, c. 1974, which had been used as the home of Radio Caroline South from 1964 to 1968 and 1972–1980
"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 was launched by Irish music entrepreneur Ronan O'Rahilly, and started broadcasting from a ship off the Essex coast in 1964. By 1967 ten pirate radio stations were broadcasting to an estimated daily audience of 10 to 15 million. Influential pirate radio DJs included John Peel, Tony Blackburn, Kenny Everett, Johnnie Walker, Tony Prince, Emperor Rosko and Spangles Muldoon.
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, landbased pirate stations took to the air on medium wave at weekends, such as Radio Free London in 1968.
By 1965 pirate radio had a British audience of 10 to 15 million, and by 1966 sold almost £2 million worth of advertising a year to customers including the government-funded Egg Marketing Board. Radio Caroline's audience was one third the size of the Light Programme in the parts of the country where it could be received, but the Light Programme's audience did not decrease, indicating that pirate radio appealed to an audience that the BBC did not serve.
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 (with some sizeable off-air periods between 1968 and 1972 and 1980–83) until 1990.