The Prime Minister is in denial about the gravity of the situation in Iraq

The tricky thing about a quagmire is you never know you have walked into one until it is too late to get back out of it. The further in before you discover it, the greater the difficulty in extricating yourself.

Yet the country is being led deeper into the mire of occupied Iraq by a prime minister who stubbornly refuses to recognise the dangers. Those who have met Tony Blair recently, whether friends or critics, have left in alarm at how little he grasps the gravity of our position in Iraq. We have a prime minister in denial.

The reality of our position inside Iraq is dire. Across most of Iraq, the Western presence is barricaded behind its fortifications of concrete and razor wire, venturing out only in armoured convoys. The military have lost control over that most basic asset of an expeditionary force - its supply routes. For a couple of days the other week the coalition presence in Baghdad was reduced to emergency rations because fresh food was not getting through. The strategy of passing the buck for security to Iraqi forces has collapsed following the discovery in Fallujah and Najaf that local soldiers and police will not die for the United States. BP and others have abandoned the country, stalling the repair of the oil industry which was expected to pay for the costs of reconstruction.

Downing Street routinely blames the problem on a few thousand terrorists, fundamentalists and extremists, but this is delusional behaviour of a high order. The diverse resistance groups across Iraq are known to be in contact with each other and may be only weeks away from the launch of a united opposition under some banner such as a popular front for the liberation of Iraq.

But the fundamental problem for the coalition partners is not security but legitimacy. The justification that they were in occupation because they had liberated Iraq was only valid for as long as their presence was welcome to the local population. From the moment that they were remaining in occupation in defiance of the wishes of the majority of the Iraqi people, they lost any claim to moral authority as liberators.

And we have certainly past that watershed in local opinion. Opinion polls now report most Iraqis wanting their occupiers to leave. To persist in occupation against the wishes of a majority of the locals is to cast ourselves in the role of neo-colonial rulers.

Part of the problem in Downing Street is that both Tony Blair himself, and the New Labour acolytes with whom he surrounds himself, are too young to remember the painful lessons of the final days of Britain's colonial period. Iraq is now providing them with a contemporary lesson in the destructive dynamic of colonialism. Unpopular occupation can only be enforced with violence but violence undermines still further the legitimacy of occupation.

The slaughter at Fallujah destroyed the credibility of US forces as liberators rather than occupiers. The worst outrage in the history of the British occupation of India was the massacre at Amritsar, in which the official number of dead was 379. (In fairness to the British Raj, they did at least count their victims, unlike the coalition forces in Iraq.) The number shot in Fallujah was double that figure, mostly women and children. Even the number of villagers massacred at My Lai during the Vietnam War was lower than in Fallujah. And yet there are still senior figures in the Pentagon who regret that they were prevented from "finishing the job" and flattening the city.

The graphic photographs from Abu Ghraib are disgusting and depraved. But they are not surprising. Brutality by the occupier against the occupied is a repeated pattern of colonialism. Nor are the prisoners the only ones to be degraded. The most chilling feature of those photographs is the happy, cheerful grins with which the tormentors went about their task of abuse. They had all too evidently assumed the supremacy of colonialists and the contempt for natives that accompanies it. That is one of the intrinsic dangers of colonialism. It brutalises the occupying armed forces who cannot admit to themselves that the people they repress by force are human beings like themselves.

There is one stark difference from previous colonial eras. In the digital age the electorate back home have been brought face to face with vivid images of the oppression that is inseparable to armed occupation. The prison photographs have destroyed the legitimacy of our presence in Iraq not only among its population, but within the British people as well.

It was a leaked Red Cross report into torture in Algiers that provided the turning point in French support for the war of occupation. It was the account of the fatal beating at Hola camp that broke British support for the colonial war in Kenya. The occupation of Iraq will inevitably end in the same way. No government, not even with the record majority of Tony Blair, can indefinitely maintain an armed occupation against local resistance in Iraq and without popular support in Britain.

Incredibly Tony Blair is not trying to find the exit door but perversely is gearing up to expand the portion of Iraq that Britain occupies. Even as you read this, British regiments are on 24 hours' notice to move to Iraq.

If this was the result of the Chiefs of Staff asking the Prime Minister for more troops to enhance the security of the British sector, no one would query it. But in truth it is the result of President Bush asking Tony Blair for 5,000 more British troops to take over from US forces in Najaf, and there are hard questions that need to be answered before we can agree.

First, are not our present problems the result of agreeing to President Bush's earlier demands that we join in his war, and when did he last listen to anything we asked of him? Nothing would relieve Labour MPs more than for Tony Blair to show a spark of independence from a US President who is uniquely unpopular in Britain.

And what will be the risks to our troops in their new area of operation? The armed resistance in Najaf has already been radicalised by the heavy-handed techniques of the US military and is not going to drop its hostility because the coalition forces they confront now wear British uniforms. Patrols in Najaf will be heavily armoured and seriously threatened. Which provokes another question. Can British forces around Najaf operate as if in a hostile environment, without the British presence around Basra ending its practice of patrolling on foot without flak jackets? If the resistance fighters in Najaf find the local British forces too well protected, will they not nip down to Basra where there are softer targets belonging to the same army?

It is over a year since Parliament voted to commit troops to the invasion of Iraq. Since then every justification for the war has collapsed, from the original failure to find any of the weapons of mass disappearance to the recent implosion of the claim that we would bring respect for human rights and democracy to the region. More British troops should not be committed without a fresh mandate from Parliament. And in the light of all that MPs have learnt since their last vote, this time Parliament should say No.