Once lost, trust is difficult to regain. Its absence has infected the Government

It is undignified for the Prime Minister to go on believing in a threat from Saddam which everyone else can see was a fantasy

Who could have predicted that the year's parting blow to No 10's case for invading Iraq would come from Paul Bremer, handpicked to run the US occupation for his tenacity for staying on message? Yet his rebuttal of Tony Blair's claim of "massive evidence of a huge system of clandestine laboratories" could not have come from a more impressive source than the head of the Coalition Authority in Iraq.

It was a fitting coup de grâce to a year in which the war in Iraq has come to dominate politics. This was not the expectation of those who got us into the war. I vividly remember a conversation with one cabinet minister who went along with the war on the basis that its unpopularity would soon be forgotten provided it was quick. Tony Blair was equally confident that a military victory would draw a line under the controversy of his decision to join in a war made in Washington.

Unfortunately the controversy dogged the Government for the rest of the year. The war in Iraq dominated 2003 and may yet be remembered as the defining issue of Tony Blair's second term. His embarrassing position in a poll as the least trusted of 30 politicians is due largely to the perception that he sold a pup to the public over Saddam's weapons of mass destruction. Once lost, trust is difficult to regain and its absence has infected the credibility of the Government.

The greatest political damage from the war in Iraq is that it totally eclipsed the domestic agenda. Yesterday Steven Byers wrote frankly about the sense of loss of momentum that suffuses public judgement on this government. There is a real danger that a Labour Government that has held office for the longest period in history with the largest ever Labour majorities will not be remembered for domestic achievements. In some ways that would be unfair.

The virtual elimination of youth unemployment and the steady progress towards eliminating child poverty are radical changes for the better. But they go unremarked in part because every commentator can sense that freeing Britain from poverty does not excite the current occupants of No 10 in the same way as liberating Iraq from Saddam.

Tony Blair has no one to blame but himself if Iraq has obscured his more benign successes. There are two main reasons why throughout the year the controversy over the war would not go away. Both of them are rooted in the justifications that were given by Tony Blair for the invasion.

We were told that occupying Iraq would be a victory in the war on terrorism. Yet, nine months after the toppling of Saddam's statue, Paul Bremer was conceding yesterday that al-Qa'ida and other terrorist networks, who were not tolerated in Iraq before we occupied it, are now behind much of the daily attacks against us, which over the weekend claimed the lives of another seven allied soldiers and rather more Iraqi policemen. Far from being a victory in the war on terrorism, the invasion of Iraq has been a spectacular own goal, as our intelligence services accurately warned the Prime Minister in advance of the invasion.

We now have a new front against terrorism within Iraq with no evidence of any reduction in terrorism outside Iraq.

Famously, we were also assured that we had to invade Iraq because its weapons represented a real and present danger to British interests. The motion the Government put before the House of Commons on the eve of war was entirely about the necessity of disarming Saddam's weapons of mass destruction. In a lengthy paragraph he did not even mention the desirability of freeing the Iraqi people, although that has become the official rationalisation of the war now that they have been unable to find any weapons to disarm.

It is not enough for No 10 now to justify the invasion of Iraq on the grounds that they have indeed found weapons of mass destruction - but in Libya. Their novel defence appears to be that their strategy was right and it was only a minor detail that they invaded the wrong country. They must not be allowed to get away with it. One of the questions they were repeatedly asked before the war was: "Why Iraq?" Why was such high, urgent priority attached to Iraq, when there were many other countries where the evidence of weapons of mass destruction was greater?

In any case, the strategy that worked in Libya was not armed force but diplomatic engagement. By the close of his interview on television yesterday Paul Bremer was claiming that it made no difference whether or not we ever found any weapons of mass destruction. They still did the right thing by invading Iraq, even if for the wrong reasons. This also cannot be allowed to pass. The doctrine which underpinned invasion was President Bush's assertion of a new right of pre-emptive strike. It is not open to those who claim the right of attack of Iraq to remove an imminent threat, now that they cannot find that threat, to tell us it does not really matter. If there was no threat, there logically could have been no right to a pre-emptive strike. The very least George Bush and Paul Bremer owe the rest of us is to bury the doctrine of pre-emptive strike - and to do it fast before another country somewhere else in the world employs it as its excuse to invade a neighbour.

Tony Blair has not yet tried to argue that it makes no difference whether we never find weapons of mass destruction, but I would almost prefer him to do so than persist in his unhealthy stated denial of the mounting evidence that they do not exist.

What is extraordinary about his latest exaggeration of Saddam's weapons capacity is that he has the self-confidence to include it in a Christmas message to troops in Iraq, who know perfectly well that they have not found any of "the massive evidence" in which their Prime Minister believes.

They, like the Iraq Survey Group, have been unable to unearth a single chemical or biological agent, a single warhead, or a single delivery system. Their officers will have seen the reports of the interrogation of the top Iraqi military who have separately but consistently told the same story: that chemical and biological weapons were destroyed in 1991 and were never produced again.

It is undignified for the Prime Minister, and worrying for his nation, to go on believing in a threat which everyone else can see was a fantasy. Nor will Tony Blair ever recover his credibility until he stops insisting he is right when the public can see he was wrong. He will only expose himself to repetitions of this weekend's debacle in which he is slapped down by his treasured Washington allies who have moved on from pretending Saddam was a threat.

This is the time for New Year resolutions. As Tony Blair makes out his list he could do worse than promise himself he will admit that he got it wrong when, in good faith, he believed that Saddam had real weapons of mass destruction. And then he could promise the rest of us that he will never again commit British troops to a pre-emptive strike on anything as imprecise as single-sourced intelligence.