British would-be jihadists are heading to the Middle East to seek terrorist training because parts of the Arab world have "once more become a permissive environment for al-Qaida", the head of MI5 has warned.

Jonathan Evans said this was a "new and worrying development and could get worse as events unfold … some will return to the UK and pose a threat here".

Though Evans did not say how many people from the UK had gone to the region, it is believed the agency has monitored more than 100 who have attempted to link up with extremists in countries such as Yemen, Egypt, Syria and Libya.

A few Britons have also travelled to the horn of Africa, in particular Somalia.

In a speech in London on Monday night, Evans said one byproduct of the Arab spring was giving al-Qaida a chance to re-emerge in the countries where the network first won popular support.

In the long-term, more democracy should "ease some of the pressures that have spawned extremism in the region", he said, but in the short-term, with the Arab world in radical transition, this "more immediate problem has emerged".

"This is the completion of a cycle," said Evans. "Al-Qaida first moved to Afghanistan in the 1990s due to pressure in their Arab countries of origin. They moved on to Pakistan after the fall of the Taliban. And now some are heading home to the Arab world again. And a small number of British would-be jihadis are also making their way to Arab countries to seek training and opportunities for militant activity."

The scale of terrorist ambition emerging from Yemen is believed to be at the top of MI5's worry list, and the director general made particular reference to recent plots believed to have been hatched in the country: "Repeated attempts by al-Qaida in Yemen to mount attacks on aircraft, as we have seen in the underpants bombs and the bomb found in a printer cartridge at East Midlands airport, could have caused mass civilian casualties."

The speech on Monday night was the first Evans has given since 2010, and attempted to put into context the counter-terrorist threat posed to the UK, and potential problems in the future.

He acknowledged that less than half of MI5's priority case work now involved some kind of Afghanistan or Pakistan dimension – a few years ago it accounted for 75% of the security agency's work.

"We appear to be moving from a period of a deep and focused threat to one where the threat is less monolithic," he said. As well as the potential for al-Qaida reemerging in Arab countries, Evans said some supporters of the al-Shabaab militia in Somalia "were seeking to work with al-Qaida in Yemen, and there are links across to Mali and down to west Africa, where the UK has political, economic and demographic ties".

Evans said Iran was another source of potential concern, as the west and Israel put pressure on Tehran to abandon any nuclear weapon ambitions it may secretly harbour.

MI5's director general said there had been a series of attempted plots against Israeli interests in India and Azerbaijan, as well as a plan by Iran's Islamic Revolutionary Guard Corps to attack the Saudi ambassador in the US.

"A return to state-sponsored terrorism by Iran or its associates, such as Hezbollah, cannot be ruled out as pressure on the Iranian leadership increases."

Evans said the Olympics was now at the forefront of his mind. He admitted that the Games were an "attractive target for our enemies … no doubt some terrorist networks have thought about whether they could pull off an attack".

But he said the Olympics was not an easy target and the agency's preparations had gone well. It is believed there has been no upsurge in threat reports in recent weeks, as had been expected.

One area of increasing concern is the threat posed by state-sponsored cyber espionage – MI5 would only get involved in such cases if such thefts or disruptions posed a potential threat to national security.

Evans said MI5 was currently investigating "cyber compromises in over a dozen companies, and are working with many others of high economic value … that are potential future targets of hostile state cyber activity".

Russia and China are understood to be behind most, but not all of the these attacks.

"One major London listed company with which we have worked estimates that it incurred revenue losses of some £800m as a result of hostile state cyber-attack. They will not be the only corporate victim of these problems."

In the months after the Olympics, MI5 is likely to reorganise itself to give more resources to counter the threats from cyber space.

"Vulnerabilities in the internet are being exploited aggressively, not just by criminals but also by states," Evans said. "And the extent of what is going on is astonishing, with industrial scale processes involving many thousands of people lying behind both state sponsored cyber espionage and organised cyber crime." Terrorist groups had not posed a significant threat in this area, he said.

"But they are aware of the potential to use cyber vulnerabilities to attack critical infrastructure and I would expect them to gain more capability in the future."