Two weeks ago, the world woke to the grim reminder that fringe terrorists in Northern Ireland still put fanaticism before innocent lives. These brutal acts, so devastating in their impact on the families of those murdered, have led the people and politicians of Northern Ireland to stand as one against any return of the terrorist threat. We should be under no illusion, however, that the biggest security threat to our country and other countries is the murderous agents of hate that work under the banner of al-Qaida.

We know that there is an al-Qaida core in northern Pakistan trying to organise attacks in Britain. We know also that there are a number of networks here and as the head of MI5 reported recently: "There is no cause for complacency; there is plenty of activity and the threat level remains at severe."

It is a measure of the challenge we face - but also our success in dealing with it - that in the last two years more than 80 terrorists who planned to kill British citizens have been convicted and are now behind bars, some under the measures we have brought in since 2001.

Al-Qaida terrorists remain intent on inflicting mass casualties without warning, including through suicide bombings. They are motivated by a violent extremist ideology based on a false reading of religion and exploit modern travel and communications to spread through loose and dangerous global networks.

We must remain vigilant at all times. On Tuesday, we will publish our updated counterterrorism strategy, showing why this vigilance remains so vital and showing also the success we have had, thanks to the hard work of the thousands of brave, skilled and dedicated people working to keep us safe, investigating terrorist activities, stopping them and bringing those responsible to justice. Of the 80 terrorists now behind bars, half of them pleaded guilty. We know this is hitting terrorists' morale as well as disrupting more than a dozen plots that could have caused hundreds of deaths.

The approach we are taking tackles the immediate threat through the relentless pursuit of terrorists and disruption of their plots, builds up our defences against attacks and our resilience to deal with them, and addresses the longer term causes - understanding what leads people to become radicalised, so we can stop the process.

Across all these strands, our response must be international, national and local. At the national level, we have built the strongest-ever counterterrorist framework, with investment rising from £1bn in 2001 to £3.5bn in 2011. At our borders, we have brought in improved electronic checks, excluded more than 150 people from Britain on national security grounds since 2005 and toughened our approach to refusing entry to extremists. We have better protection of crowded places, major buildings and our transport system and we are setting out a new approach to the risk that terrorists will abuse modern technology to mount chemical, biological, radiological or nuclear attacks.

Over recent years, we have set up a national security committee involving the heads of agencies and the armed forces, and a national security forum bringing in the best outside experts. We have built up not only our national policing capability but also counterterrorist police in the regions.

But today, not only the police and security and intelligence officers, and our armed forces, but also the emergency services, local councils, businesses, and community groups are involved in state-of-the-art civil contingency planning.

Tens of thousands of men and women throughout Britain - from security guards to store managers - have now been trained and equipped to deal with an incident and know what to watch for as people go about their daily business in crowded places such as stations, airports, shopping centres and sports grounds.

This is not just about training and equipping professionals, however. I believe that the better we inform the public, the more vigilant the public will be. And there is a duty on all of us - government, parliament, and civic society - to stand up to people who advocate violence and preach hate, to challenge their narrow and intolerant ideology - in public meetings, in universities, in schools and online.

This is also a global challenge and Britain is at the forefront of international co-operation - from helping Pakistan investigate the murder of Benazir Bhutto to the work of our armed forces in Afghanistan and, in the longer term, our aid programme and our support for conflict-prevention and stabilisation.

In 2001, al-Qaida were based in Afghanistan. While they are still active there, core al-Qaida has shifted across the border into Pakistan. More than two-thirds of the plots threatening the UK are linked to Pakistan. Together with the new US administration, we are developing a strategy to tackle the terrorist threat across the region, the underlying causes, the extremist madrasas and the lawless spaces in which terrorists recruit or train. A vital part of this is building up the security forces of Afghanistan and Pakistan, with the support of our own armed forces, so they can take on more of this responsibility for themselves, an approach I discuss frequently with Presidents Karzai and Zardari, urging closer co-operation between their countries.

As the threats we face are changing rapidly, we can never assume that the established way of doing things will be enough. We will always make the necessary changes, whether through greater investment, changes to our laws or reforms to the way we do things, to ensure that Britain is protected.

And at all times, the responsibility remains the same - protecting the security of all and safeguarding the rights and freedoms of the individual. I outlined to Parliament last week the steps we are taking to make absolutely clear that we meet the highest standards, continuing to condemn unequivocally the use of torture, never torturing nor ever asking others to torture for us.

Terrorism threatens the rights that all in this country should hold dear, including the most fundamental human right of all - the right to life. We know that terrorists will keep on trying to strike and that protecting Britain against this threat remains our most important job.

I believe that this updated strategy, recognised by our allies to be world-leading in its wide-ranging nature, leaves us better prepared and strengthened in our ability to ensure all peace-loving people of this country can live normally, with confidence and free from fear.