VoIP services could be threatening the security of the UK, says the head of MI5.

According to Jonathan Evans, the danger with online calls is that they do not result in telephone bills, which are key evidence documents in prosecutions. This means it would be much easier for terrorists to make the calls and eventually escape prosecution if they are tried for criminal offences.

Evans, director general at MI5 since 2007, said online phone calls posed a "significant detriment to national security" by enabling terrorists to communicate with less risk.

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Evans also championed the government's planned 'super database' of every phone call, website visit and email in the UK, saying that it was important the security agencies could access this information.

"If we are to maintain our capability we are going to have to make decisions [on powers to intercept communications] in the next few years," Evans told the Daily Telegraph, during the MI5's first-ever interview with the British media. "Because traditional ways are unlikely to work."

It was recently revealed that the Government plans to allow an external firm to handle the database as part of a cost-saving exercise.

See also: VoIP conversations vulnerable to evesdropping