Jonathan Evans claims the London Olympics will not be an easy target (Reuters)

The director-general of MI5 has admitted it is impossible to guarantee the safety of the London Olympic Games.

In a public speech one month before the games begin, Jonathan Evans said there was no doubt that the Olympics represented a significant target for terrorist groups - but not an easy one.

"The games are not an easy target and the fact that we have disrupted multiple terrorist plots here and abroad in recent years demonstrates that the UK as a whole is not an easy target."

He made it clear that the threat to the games was hard to quantify, adding: "The preparations have gone well but planning for the future is always planning for uncertainty.

"We are far from complacent. A lot of hard work lies ahead and there is no such thing as guaranteed security."

Evans' admission that the games are an "attractive target" for terrorists will surprise few. The country was made aware of the level of security being put in place when it was announced that London would be protected by 13,500 troops, fighter jets and anti-air missiles.

He said: "Those of us who are paid to think about the future from a security perspective tend to conclude that threats are getting more complex, unpredictable and alarming. After a long career in the security service, I have concluded that is is rarely the case.

"The truth is that the future always looks unpredictable and complex because it hasn't happened yet. We don't feel the force of the uncertainties felt by our predecessors. And the process of natural selection has left us, as a species, with a highly developed capacity to identify threats, but a less developed one to see opportunity.

"This helps explain the old saying that when intelligence folk smell roses they look for the funeral."

Evans also told the Lord Mayor's annual defence and security lecture that a significant threat facing the games comes from a British would-be jihadis who go to the Arab world for military training, which he described as a "new and worrying development".

Evans went on to claim that the removal of UK troops from Afghanistan would represent an important point of transition when it comes to fighting terrorism.

"The intervention in Afghanistan in 2001 was forced on the West because the Taliban regime was affording al-Qaida the base it needed from which to plot catastrophic terrorism against us.

"It will remain important that terrorism can still be countered there after 2014, even if counter-insurgency becomes less prominent."