The head of Britain's internal spy service said on Friday that the country faced renewed threats from terrorism and his organization was fighting an "invisible struggle" with al-Qaida.

The head of MI5 Jonathan Evans said in a speech in London that "there is a persistent intent on the part of al-Qaida and its associates to attack the UK."

He added: "The country continues to face a real threat from al Qaida-related terrorism. That threat is diverse in both geography and levels of skill involved but it is persistent and dangerous and trying to control it involves a continual invisible struggle. Counter-terrorist capabilities have improved in recent years but there remains a serious risk of a lethal attack taking place. I see no reason to believe that the position will significantly improve in the immediate future."

MI5 is the secret service with the job of guarding against terror attacks in Britain. Evans, who has been the director- general of MI5 for just over three years, is an expert on al-Qaida.

He said there was still a threat from al-Qaida based in the border areas but new al-Qaida threats were coming from Somalia where "there are a significant number of UK residents training in al Shabaab camps to fight in the insurgency there."

London will be hosting the Olympics in the summer of 2012, and it presents a tempting target for terrorists, warned Evans.

"We are now less than two years from the London Olympics. The eyes of the world will be on London during the Olympic period and the run up to it. We have to assume that those eyes will include some malign ones that will see an opportunity to gain notoriety and to inflict damage on the UK and on some other participating nations," Evans said.

He said there will be a major security operation to support the Olympics, but was cautious of creating too optimistic an atmosphere and said "we should not underestimate the challenge of mounting the Games securely in an environment with a high terrorist threat, the first time this has been attempted."

Evans turned to other terrorist threats, besides al-Qaida and said there had been a "persistent rise in terrorist activity and ambition" in Northern Ireland over the last three years.

He said there had been more than 30 attacks or attempted attacks so far this year by dissident Republicans on national security targets compared to just over 20 for the whole of last year, ranging from shootings to under-car devices to large vehicle bombs.