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Young people are increasingly turning to virtual private networks (VPNs) to anonymise their free sharing of music and movies, a new study has suggested.

Sweden's Lund University indicated that there had been a 40% rise in the number of 15 to 25-year-olds using such services since 2009.

Many believe a clampdown on piracy is behind their rise in popularity.

The Pirate Bay has advised visitors to make use of VPNs. It would be illegal to do so to download pirated files.

In its first public response since five of the UK's big ISPs agreed to block their subscribers from the file-sharing site, The Pirate Bay remained defiant.

"As usual there are easy ways to circumvent the block. Use a VPN service to be anonymous and get an uncensored internet access, you should do this anyhow," it said.

Some industry experts now believe that VPNs could become publishers' next target.

Once the preserve of the business world, VPNs are secure networks that allow data sharing behind heavily encrypted firewalls.

The fact that they allow users to swap files without being detected makes them perfect for pirates.

"VPNs could become the next front in the battle against piracy," predicted independent music analyst Mark Mulligan.

He pointed to the growing popularity of VPNs such as BT Guard - in this case, BT stands for bit torrent not British Telecom.

Increasingly services such as the bluntly named HideMyAss have been taking extra measures to protect their users, he added.

"Some providers have already starting putting anonymity systems in place, such as not tracking IP addresses and deleting logs after seven days."

The music industry has changed its focus over the last year, away from targeting individual file-sharers to shutting off access to sites via domain name service blocking - meaning anyone typing in the address of a torrent site will not get through.

To achieve this, content providers must come to an agreement with internet service providers to block access or force the block via the courts.

Crackdowns against The Pirate Bay have now been enforced across Europe and are imminent in Britain.

Some have questioned the effectiveness of current blocks.

BT, for example, has adapted its child abuse filtering system known as Cleanfeed, but has made no secret of the fact that the system is not entirely foolproof.

Use of so-called proxy servers in conjunction with a VPN is one way to circumnavigate the filters.

"BT's Cleanfeed is the Rolls Royce of filtering software but there are always ways around it," said Mr Mulligan.

Data collected by the music industry body the IFPI (International Federation of the Phonographic Industry) suggests blocking has had a significant impact.

Blocks against the The Pirate Bay in Belgium reduced the service's audience by 84% between August and November 2011, according to Comscore.

In Italy, usage of the service is down by 74%, according to a Nielsen study commissioned by the IFPI. Use of BTjunkie, another torrent service blocked in Italy, was down by 80%.

The fact that young music fans are moving to VPNs signals something of a victory for the music industry, thinks Mr Mulligan.

"The aim of such blocking is not to turn off the tap but to make it as inconvenient as possible to get to such services," he said.

"VPNs add an extra layer of complexity and young people have to pay £5 or £6 a month to use them, which means some of the reasons for doing it are lost."