Gordon Brown will tonight vigorously defend Britain's mission in Afghanistan and Pakistan, warning that al-Qaeda remains the biggest threat to Britain's national security.

Speaking to the Lord Mayor's Banquet at Guildhall, the Prime Minister will say that the terrorist network has an extensive recruitment operation across Africa, the Middle East, Western Europe and the UK. He will add that there are several hundred foreign fighters based in Pakistan travelling to training camps to learn bomb-making and weapons skills. Al-Qaeda also has links to the Afghan and Pakistan Taleban.

He will remind his audience that since 2001, nearly 200 people have been convicted of terrorist or terrorist-related offences in Britain. "And to those who say this threat is not real, I ask them to consider that almost half of those convicted pleaded guilty.”

In a defiant message to the doubters he will say:“Vigilance in defence of national security will never be sacrificed to expediency. Necessary resolution will never succumb to appeasement. The greater international good will never be subordinated to the mood of the passing moment.”

He will add:“So I vigorously defend our action in Afghanistan and Pakistan because al-Qaeda is today the biggest source of threat to our national security — and to the security of peoples lives in Britain … and tonight I can report that more has been planned and enacted with greater success in this one year to disable al-Qaeda than in any year since the original invasion in 2001.

“Tonight I want to leave you with a clear summary of Britain’s case, and that of the coalition as a whole. We are in Afghanistan because we judge that if the Taleban regained power al-Qaeda and other terrorist groups would once more have an environment in which they could operate. We are there because action in Afghanistan is not an alternative to action in Pakistan, but an inseparable support to it.”

Mr Brown will go close to breaking convention at the City event by taking a swipe at David Cameron's Europe policy, with a warning against isolationism.

"At every point in our history where we have looked outwards, we have become stronger. And now, more than ever, there is no future in what was once called 'splendid isolation'.

"When Britain is bold, when Britain is engaged, when Britain is confident and outward-looking, we have shown time and again that Britain has a power and an energy that far exceeds the limits of our geography, our population, and our means.

"As a nation we have every reason to be optimistic about our prospects: confident in our alliances, faithful to our values and determined as progressive pioneers to shape the world to come."

Withour mentioning the Conservatives, whom he has attacked for leaving the main centre-right grouping in Europe, he will add:"And that is why I say our foreign policy must be both patriotic and internationalist: a foreign policy that recognises and exploits Britain's unique strengths, and defends Britain’s national interests strongly – not by retreating into isolation, but by advancing in international co-operation."

Times Recommends

• UK regional accents thrive against the odds
• ‘Scarecrow’ wind farms put rare birds to flight
• What an anti-climax: G-spot is a myth