To go by appearances, Jonathan Evans could be a genial senior manager on a dress-down workday. This is MI5, but not as Britain has ever known it.

Where once there may have been starched shirts and stiff gins, there are now soft collars, coffee and custard-cream biscuits. Even the chatter, if carefully worded, has the air of the informal board meeting.

Mr Evans, MI5’s new chief, invited reporters into the understated heart of the domestic Security Service community yesterday.

The first interview given by a serving head of MI5 covered everything from al-Qaeda and the IRA to the rather animated portrayals of agents in the television series Spooks.

He disclosed that al-Qaeda’s high command in Pakistan remained intent on using British citizens to carry out attacks on British soil, making necessary the constant surveillance of thousands of suspects. Terrorists were being forced “to keep their heads down” because of relentless surveillance and successful prosecutions.

The chat with the Director-General — arranged to mark MI5’s centenary year — revealed more of the role of the intelligence agency and its myriad projects. More than 40 per cent of his staff are female, the average age is 40, and 8 per cent are of ethnic minority origin. Current concerns include the internet indoctrination of teenage Muslims into terrorism and the rise in dissident Irish republican attacks.

The MI5 chief also divulged that there had been indirect connections between the terrorists in Mumbai in November and individuals in Britain but that there was no evidence of any national security issue for Britain.

It is understood that billing records have been uncovered revealing telephone calls between the Pakistan-based Lashkar-e-Taiba group suspected of being behind the Mumbai attacks and other countries, including Britain. “But nothing of security significance was found,” Mr Evans said.

Breaking with secrecy traditions to speak of his concerns, Mr Evans also said that 2,000 terrorist suspects in Britain were the subject of constant surveillance. “The strategic intent of the al-Qaeda core, [based] in Pakistan, is to mount attacks in the UK, and their model is to use British nationals or residents to deliver the attacks,” Mr Evans told newspapers invited into MI5’s headquarters in London.

However, in the past 18 months there had been fewer cases where terrorists had moved from facilitating and supporting terrorism to planning attacks. He said: “There have been 86 successful convictions since January 2007 of whom approaching half pleaded guilty, which has had a chilling effect on the enthusiasm of the networks. They’re keeping their heads down.”

Mr Evans, 50, a classical studies graduate of the University of Bristol, also made clear that his job bore little resemblance to that portrayed in Spooks. The BBC television actors are better dressed and “more omnipotent”, and more prone to histrionics.

The recession could have a long-term impact on Britain’s national security, making the country more vulnerable to terrorism, espionage and radicalism, he suggested. History had shown that previous worldwide recessions had had worrying repercussions. The security threat would depend on whether the downturn proved to be a “watershed moment”, affecting British society on a much larger scale than was now the case. Although there was no direct relationship between economic distress and extremism, the security repercussions should the West become less economically dominant had to be kept in mind.

Mr Evans admitted that MI5 had experienced “a tumultuous period”. The expanding MI5 organisation — staffing levels are to rise to 4,100 by 2011 — had focused on monitoring terrorist suspects, and the main targets had been forced to adopt varied counter-surveillance techniques to avoid being followed or bugged. “My staff are on the streets every day trying to keep the nation safe,” he said.

There was a “capability war” going on between the terrorists and MI5. Al-Qaeda suspects never spoke to each other in or near buildings, and learnt lessons from court cases where surveillance methods were disclosed during the prosecutions. Terrorist recruits were also using circuitous routes, such as Dubai, to get to places such as Afghanistan to join the jihad.

Of the use by terrorist organisations of the internet to try to recruit young Muslims in Britain, he said: “It’s a form of child abuse, trying to exploit young people over the internet.” They became vulnerable to al-Qaeda propaganda that sought to take advantage of dramatic images of people suffering in Iraq, Afghanistan, and now from the Israeli military attacks in Gaza, which he said could increase radicalism.

“There is no single path leading to violent extremism,” he said. “There are varying factors including social, foreign policy, economic and, often, personal reasons which persuade them to throw in their lot with the extremists.”

The MI5 chief also said that the 2012 Olympics in London potentially presented a major terrorist target but the right way to deal with it was not to pursue every red herring but to concentrate on known terrorist networks.

And as for the ordering of assassinations, MI5 is emphatic. Whatever happens in Spooks, the MI5 website states categorically that it “does not kill people or arrange their assassination”.

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