The Olympic Games are an attractive terrorist target, MI5 spy chief Jonathan Evans has warned.

The reclusive head of Britain's clandestine intelligence service also revealed the country was also battling an astonishing level of cyber attacks on government and industry systems, revealing one company -- possibly defense contractor BAE Systems [LSE:BA] -- has suffered a staggering £800 million in lost contracts and damages after being hacked.

Other threats outlined in Evans' inaugural Lord Mayor's Annual Defence and Security Lecture in London on Monday night included Al Qaeda's infiltration of Arab Spring countries and potential extremists motivated by the possible collapse of the euro zone.

Speaking about the level of computer hacking faced by the UK, Evans said: The extent of what is going on is astonishing.

Industrial-scale processes involving many thousands of people lying behind both state-sponsored cyber espionage and organized cyber crime.

Vulnerabilities in the Internet are being exploited aggressively, not just by criminals but also by states.

The Olympics, Evans added, were a particularly attractive target for terrorists, but so far the threat level remained at substantial - one level below severe, where it has stood for years.

As part of preparations for the games, MI5 -- the agency who employ fictional spy James Bond -- have vetted half a million people, with a number of individuals turned down for accreditation due to security concerns.

But while the games were an attractive target, Evans said, they would not be 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.