Internet telephone services pose a serious threat to Britain's security, the head of MI5 has said.

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

In an interview with the media Wednesday, 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.

Evans lent his support to the government's planned database of every phone call, internet use 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," he was reported as saying, in the Daily Telegraph. "Because traditional ways are unlikely to work."

He was speaking to a small audience of newspaper journalists on the centenary of MI5's creation. It was the first time in MI5's history that a serving head of MI5 had interviewed with the press.

Evans underscored that Israel's attack on Gaza could inspire further extremism and warned that the economic fallout from the global credit crunch may bring fresh security risks.

But critics contend that allowing the government to create such a "super database" raises privacy concerns as well as potential security problems over how the data would be stored.

It emerged in recent weeks that the government has been looking for a private firm to run the database.