There are more than 150 illegal stations across the country, a third of which are said to be run by criminal gangs who use them as a front to sell drugs. Previous raids have found drugs, guns and ammunition among the piles of CDs.

Drug dealers within earshot of some stations keep tuned in to wait for a particular song to be played or phrase to be uttered, knowing that it is the signal that their next shipment is ready for collection.

The stations started in the 1960s with Radio Caroline, which played records for teenagers from the middle of the North Sea.

Now the underground industry is big business. The stations can make up to £10,000 a week for their owners, who go to great lengths to protect their operations, many employing armed bodyguards.

Some stations charge DJs for each slot they use, take money from local businesses who do not realise that the stations are illegal, and also sell drugs at parties they advertise.

Officers from Ofcom, which polices the country’s airwaves, have found doors wired to mains electricity when they have raided studios to take down aerials.This week Ofcom and the Metropolitan Police shut down 44 stations by removing their aerials, which are often miles from the operating bases.

Robert Thelen-Bartholomew, Ofcom’s head of field operations, said: “Illegal broadcasting affects safety of life services and has links with serious crime. Ofcom will continue to pursue and prosecute those involved in this criminal activity.”

Some of the stations broadcast to a small area — a few streets — while others have the signal strength to reach out across cities. As quickly as officials tear down their equipment it goes up again.

The pirates’ signals can interfere with radio broadcasts by the emergency services and air traffic control, and also cut out legitimate stations, costing them listeners and millions of pounds in advertising.

London has about half of the country’s pirate stations, with names such as Klimaxx, Powerjam, Ruud Awakening, Ouch FM and Lightening, all of which have been raided by Ofcom in the past.

Birmingham is also a popular area, with stations that include StingFM and HotFM, which were both linked to the airing of a rape allegation that led to two nights of rioting in the Lozells area of the city last month, during which one person was stabbed to death. In response to claims that pirate radio stations service the needs of the community and give people a voice, a new type of licence, the community radio licence, has been introduced for non-profit organisations. Forty licences have been issued this year.

An Ofcom spokesman said: “We are aware of certain criminal families that run pirate stations as part of their network of illegal activities.

“They get £5,000 a week in untaxed revenue and they can also use the station to promote other events they are involved in. There are strong links between some nightclubs and pirate stations.”

One pirate station has a website that boasts of its history and professionalism in reaching “a wide variety of people by supplying them with the music they want to hear . . . the very best in Drum and Bass, Jungle, Old Skool, UK Garage, Vocal, Grime, and Sublow, and many more styles of music. Based in London, we will provide to the whole of London and surrounding areas.”

Despite its best efforts, Ofcom knows that it is almost impossible to win the war against the pirates. Within hours of a station being shut down it can be up and running again. Also, the stations warn each other when the Ofcom vans are on the prowl.

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