By Nick Higham BBC media correspondent

Many Radio 1 DJs first worked on pirate stations

The number of active pirate stations identified by the department's Radiocommunications Agency (soon to be swallowed up by the new communications super-regulator Ofcom) fell from 248 in 2001 to 209 last year.

Perhaps as a result, the number of raids conducted by the agency's inspectors also fell, from more than 1,400 to just over 1,000, but successful prosecutions rose from 20 to 49 (a 100% conviction rate).

In recent years the agency has become more sophisticated in the battery of legal devices it uses to force pirates off the air.

If you actively support a pirate station in some way you are now likely to find your telephone cut off.

Nightclubs

More than 100 warning letters were sent to businesses, mostly clubs and record shops, which promote stations.

(The maximum penalty for aiding and abetting a pirate broadcaster is two years in prison or an unlimited fine).

Seven club nights promoted by pirate stations were cancelled after pressure from the agency, and so too was a party flight to clubs in Greece which a pirate station was sponsoring.

Even so the courts are scarcely draconian in the penalties they impose: the average fine for conviction fell from �538 to �324.

But the agency's success is only relative.

Dance music has filtered into the mainstream

There are still plenty of pirate stations around: I counted at least a dozen on FM in London last Sunday lunchtime.

Despite a combination of sticks (prosecutions, fines and raids) and carrots (more licenses for legitimate stations, including "access radio" and temporary "restricted service" stations), the problem of broadcast piracy has been an intermittent thorn in authority's side for more than 40 years.

Pirate broadcasting is of course indefensible.

Although the offshore pirates of the 1960s initially took advantage of a loophole in the law, broadcasting from outside territorial waters, the 1968 Marine Broadcasting Offences Act rendered them illegal by making it an offence to work for, supply, equip or advertise on them.

Suspicions

Ever since then, pirate broadcasting, whether on land or at sea, has been unambiguously against the law.

Pirate transmissions can be dangerous.

They cause interference to other users of the radio spectrum (including air traffic control and emergency services) and swamp the signals of legitimate stations.

They may steal listeners from licensed broadcasters who pay good money for their right to transmit, and bear all the usual costs of running legitimate companies.

Peel and Blackburn

There are suspicions that some pirate stations may have links to organised crime.

The difficulty is that much of today's legitimate radio scene owes its character to the pressures exerted by previous generations of pirates - and many mainstream broadcasters, from Tony Blackburn and John Peel onwards, cut their teeth on illegal stations.

The pirates of the 1960s forced the creation of BBC Radio 1 and (later) of legal commercial radio.

Another wave of pirates in the 1980s led to the licensing of more commercial stations, producing US-style competition for audiences and services for ethnic minorities and other special interests.

Those 1980s stations were run by a bizarre range of people: anarchists and Marxists, Thatcherite free marketeers, displaced radio hams, ethnic minority community activists and oddballs like the man in Twickenham who ran a station for the old and the lonely from his suburban bedroom.

Profound effect

Many of today's successful licensed stations like Sunrise Radio and Kiss FM have their antecedents in pirate radio.

Most of today's pirate stations are expressions of dance music culture - deliberately provocative, anti-establishment, anti-authority.

Dance music has been part of the cultural mainstream for several years.

Who is to say that today's pirate stations, like the culture they reflect, won't one day migrate into the mainstream - and have as profound an effect on the commercial radio of tomorrow as their predecessors?