Tonight, British servicemen and women are engaged from air, land and sea. Their mission: to remove Saddam Hussein from power, and disarm Iraq of its weapons of mass destruction.

I know this course of action has produced deep divisions of opinion in our country. But I know also the British people will now be united in sending our armed forces our thoughts and prayers. They are the finest in the world and their families and all of Britain can have great pride in them.

The threat to Britain today is not that of my father's generation. War between the big powers is unlikely. Europe is at peace. The cold war already a memory.

But this new world faces a new threat: of disorder and chaos born either of brutal states like Iraq, armed with weapons of mass destruction; or of extreme terrorist groups. Both hate our way of life, our freedom, our democracy.

My fear, deeply held, based in part on the intelligence that I see, is that these threats come together and deliver catastrophe to our country and world. These tyrannical states do not care for the sanctity of human life. The terrorists delight in destroying it.

Some say if we act, we become a target. The truth is, all nations are targets. Bali was never in the frontline of action against terrorism. America didn't attack al Qaida. They attacked America.

Britain has never been a nation to hide at the back. But even if we were, it wouldn't avail us.

Should terrorists obtain these weapons now being manufactured and traded round the world, the carnage they could inflict to our economies, our security, to world peace, would be beyond our most vivid imagination.

My judgment, as prime minister, is that this threat is real, growing and of an entirely different nature to any conventional threat to our security that Britain has faced before.

For 12 years, the world tried to disarm Saddam; after his wars in which hundreds of thousands died. UN weapons inspectors say vast amounts of chemical and biological poisons, such as anthrax, VX nerve agent, and mustard gas remain unaccounted for in Iraq.

So our choice is clear: back down and leave Saddam hugely strengthened; or proceed to disarm him by force. Retreat might give us a moment of respite but years of repentance at our weakness would, I believe, follow.

It is true Saddam is not the only threat. But it is true also - as we British know - that the best way to deal with future threats peacefully, is to deal with present threats with results.

Removing Saddam will be a blessing to the Iraqi people. Four million Iraqis are in exile. Sixty per cent of the population are dependent on food aid. Thousands of children die every year through malnutrition and disease. Hundreds of thousands have been driven from their homes or murdered.

I hope the Iraqi people hear this message. We are with you. Our enemy is not you, but your barbarous rulers.

Our commitment to the post-Saddam humanitarian effort will be total. We shall help Iraq move towards democracy. And put the money from Iraqi oil in a UN trust fund so that it benefits Iraq and no one else.

Neither should Iraq be our only concern. President Bush and I have committed ourselves to peace in the Middle East based on a secure state of Israel and a viable Palestinian state. We will strive to see it done.

But these challenges and others that confront us - poverty, the environment, the ravages of disease - require a world of order and stability. Dictators like Saddam, terrorist groups like al-Qaida, threaten the very existence of such a world.

That is why I have asked our troops to go into action tonight. As so often before, on the courage and determination of British men and women, serving our country, the fate of many nations rests.

Thank you.