About to leak a copy of the new Arctic Monkeys album on the net? Don’t bother unless you’re willing to risk a knock on the door from the Web Sheriff.

Free music file-sharing first brought the Sheffield rockers to prominence. But when you have a million-selling album under your belt, letting your songs spread like a virus over the web doesn’t seem so cool.

Security surrounding Favourite Worst Nightmare, the hotly anticipated follow-up, is such that music writers must surrender to Arctic Monkeys’ London HQ for an advance hearing.

Now cutting-edge bands who built a following by letting fans trade MP3 song files are secretly hiring web enforcers to track down and remove unauthorised leaks that might destroy album sales.

The web detectives trace the source of a leak and individuals found to have deliberately spread copyright-protected material receive a personal visit followed by legal threats.

Indie bands don’t come much cooler than Bloc Party, the London quartet tipped to emulate Radiohead’s global success. But when a copy of their second album, A Weekend in the City, leaked online three months before its release in February they faced disaster.

It has emerged that V2, the band’s record company, hired Web Sheriff, a London web detection agency with a track record of removing material including illegal pornography and terrorism videos from the net. John Giacobbi, Web Sheriff managing director, said: “They were looking at up to one million illegal MP3 song files spreading of an album which was supposed to break the band internationally.”

Web Sheriff’s team of sleuths monitored Bloc Party fan sites and appealed to users not to invite links to the leaked album out of respect for the band.

Where the carrot did not work, the company tracked down widespread uploaders of music through internet service providers and issued letters referring to possible prosecution.

According to V2, illegal downloads of the album were reduced to a commercially insignificant level within three months. The album went gold on its British release and entered the US Top 20.

Bloc Party’s reputation as a web-friendly underground band was maintained while the Web Sheriff, which has performed a similar service for George Michael, performed the dirty work.

Beth Appleton, V2 head of new media, said: “Working together with Web Sheriff we managed to contain the distribution of the album whilst continuing to raise exposure for the forthcoming release and maintaining excitement amongst the fans.”

More than a hundred file-sharers in Britain have accepted fines of up to £6,500 imposed by the British Phonographic Industry in the battle against piracy. But the greatest challenge is posed by international operations such as the Russian allofmp3.com site which sells unauthorised copies of chart albums for less than £1.

Mr Giacobbi said: “We tracked down one massive uploader to an apartment in Moscow using traditional gum-shoe methods. They were surprised to receive a knock on the door. We told the guy next time it will be the ‘real’ sheriff. The site was closed down next day.”

The Web Sheriff is now being used to remove leaked episodes of television shows from the net. The company was hired to track down the source of distressing video clips posted on an Islamist site that showed the execution of the British hostage Ken Bigley.

Out of tune

12bn
Estimated number of songs swapped or downloaded illegally in 2006

£325m
Estimated annual loss to British record companies

2.8bn
The number of illicit MP3 files traded on Napster

£100m
Damages paid by Kazaa, a source of illicit music and film downloads, to the world’s four leading music companies in compensation for lost sales

Source: Times database

Times Recommends

• Lies and cover-up at MG Rover exposed by report
• High oil prices push manufacturing costs up
• Primark founder steps down as chief executive