So many of us must shoulder the blame for this disastrous war

I have become increasingly angry with myself for not convincing Blair of the damage he would do if he persisted in unilateral war

It was always evident to me that the media calculation in Number 10 was that no Prime Minister ever did badly out of winning a military victory. The war in Iraq would be short, victory would be absolute, and the nation would be grateful to its war leader. Hence the plonking, embarrassing photo call in Basra of Tony surrounded by liberated children singing nursery rhymes.

The nation, though, is proving stubbornly ungrateful. It did not believe there was a case for war before it started, and has seen nothing since it finished to demonstrate that its scepticism was misplaced. This week's opinion poll showed that the majority who believe the war on Iraq was unjustified is back to pre-war levels. This need surprise no one outside Number 10, as none of its justifications for war has withstood contact with reality inside Iraq.

Take for a start the lurid claims of weapons of mass destruction, ready on a hair-trigger alert to pose "a current and serious threat to UK national interests". The Iraq Survey Group apparently has found no such weapons. For good measure they have also found no chemical agents, no biological agents, no weapon laboratories and no delivery systems. This is less than even I had expected Saddam to possess. I did predict in my resignation speech that Saddam probably had battlefield chemical munitions, although I did not myself recognise these as true weapons of mass destruction constituting a current and serious threat. In the event I appear to have overstated Saddam's weapons capacity. The cupboard is totally bare.

We are warned that so far the information on this report is only a leak and that we should have patience and wait on the authorised version. Patience is not a charity that was extended to Hans Blix, who was brushed aside by those keen on invasion after less time on the ground in Iraq than has been enjoyed by the Iraq Survey Group. But in any case there is a basic dishonesty in the pretence that the Iraq Survey Group is looking for weapons. The truth is that the search party went in with the US forces, and was known, in a classic of military jargon, as the 75th Exploitation Task Force. By June they had searched every ammunition dump in Iraq and retired baffled from the hunt for chemical or biological weapons. As one of their officers expressed it in a perfect American idiom, "We came loaded for bear country, but we found no bears."

The deployment of the Iraq Survey Group did not mark the start of the search for weapons of mass destruction, but the abandonment of it. Their job has never been to find actual weapons but to come up with the evidence that Saddam had ambitions to acquire them. But we always knew that. It was precisely to thwart those ambitions that the UN pursued a strategy of containment, and everything we have learnt since entering Iraq has confirmed what a success that strategy had been.

It was also claimed, notably by President Bush, that the attack on Iraq was justified as a battle in the War on Terrorism. Donald Rumsfeld used to claim there was "bullet-proof evidence" of a link between Iraq and al-Qa'ida. But this justification also has disintegrated. Only last week President Bush was obliged to admit that there was no evidence, bullet-proof or otherwise, to link Saddam to 11 September.

Even the keenest advocate of the war would be hard put to claim that it has diminished terrorism. Indeed bomb attacks and land-mine ambushes are now so commonplace in Iraq itself that President Bush has designated Iraq the new Central Front in the war against terrorism, although this is the same country which he had previously trumpeted as the scene of a victory against terrorism.

The US neo-conservatives can be relied on to put a brave face on the failures of their strategy. Some of them now claim the increased terrorism in Iraq proves how clever they were to invade. Their reasoning, if that is the right term, is that the US presence in Iraq is the "fly-paper" which diverts terrorists from elsewhere. With talk like that back home, it is not surprising that US units within Iraq are developing a mutinous mood.

There is in any case no evidence whatever that terrorist atrocities anywhere else in the world have diminished since the fall of Baghdad. On the contrary, the invasion has resulted in increased hostility to the West in the very countries whose support we most need if we are to defeat terrorism. One recent survey of public opinion across the globe came to the blunt conclusion "the bottom has fallen out of support for America in most of the Muslim world".

The more difficult it has proved to find weapons of mass destruction in Iraq, and the more alarmingly easy it has proved to find terrorists there, the more the war party have fallen back for their justification on the fact that they rid the world of a brutal dictator. This was not the case that they made before the war, for the very good reason that it is illegal in international law to invade a country to change its government.

That is precisely why the opinion of the Attorney General on the legal basis for war is based purely on the case for disarming those weapons of mass destruction that we cannot find. Intervention on humanitarian grounds would be legal, indeed would be a duty, if it came with the multilateral support of the Security Council, but the authority of the UN has also been badly damaged by the war on Iraq. As Kofi Annan said this week, "It could set precedents that resulted in a proliferation of the unilateral and lawless use of force."

As the war emerges as deeply damaging both in international diplomacy and in domestic politics, there is an increasing tendency to blame it all on Tony Blair. The press have already started to write that up as a major theme of next week's Labour conference. It is true that the dynamic for British participation came from Number 10 and was driven by Tony's determination to maintain the special relationship with George Bush. But it is an evasion to let Tony take all the blame. Others had the chance to save him from himself.

He was meticulous in letting the Cabinet debate Iraq, but was never challenged by a sizeable number of senior ministers. He agreed, in an historic precedent, that Parliament should vote on the war before troops were committed, and Parliament voted to go along with him. The Opposition did not oppose, but constantly egged on the Prime Minister to go to war.

All of must accept responsibility for the war - and I include myself. As I have observed the dramatic revelations from two months of Hutton, I have become increasingly angry with myself for not being more persuasive at repeated meetings in convincing Tony Blair of the damage he would do to himself and his party if he persisted in a unilateral war.

There is a profound lesson from this débâcle for our constitutional system. We need an urgent return to more collegiate government, with the Cabinet and Parliament providing the real checks and balances on the preoccupations of the Prime Minster, which they failed to supply over Iraq.