In villages and monasteries in northern Iraq, and in churches in Baghdad, Irbil and Mosul, it is still possible to hear Assyrian Christians talking and praying in ancient Aramaic, said to be the language of Christ. Fewer in number now, the Assyrians are the direct descendants of the empires of Assyria and Babylonia, the original inhabitants of Mesopotamia. The Church of the East, currently presided over by Archbishop Gewargis Sliwa in Baghdad is the world's oldest Christian church.

Before the toppling of Saddam Hussein, the Iraqi Christian population numbered some one and half million. By and large, Saddam's Ba'athist government didn't discriminate against the country's minorities; indeed, Iraq's veteran Foreign Minister, Tariq Aziz was the most visible of the country's Christians. Today, barely 400,000 remain, with church leaders claiming that organised ethnic cleansing is taking place, unchallenged. Iraq's Christians have in the past been accused of collaborating with Britain and America, and while both Sunni and Shia political leaders say they want Iraq's Christians to remain, some church leaders are urging their remaining flock to abandon Iraq before it is too late and they are massacred.

If al-Qa'ida has its way, this ancient culture and people will soon be no more. In recent days, grenade and bomb attacks killed two more Christians and injured 18 more in Baghdad. Motorcyclists drove down streets, targeting Christian homes. Back in October, suicide bombers attacked the Church of Our Salvation in Baghdad, killing 58, before – and this was unreported at the time – grotesquely blowing themselves up, along with a child hostage, at the altar. In a statement afterwards, al-Qa'ida said: "Christians are a legitimate target."

Tensions between Muslims and Christians are not confined to Iraq: yesterday morning, at least seven people were killed and 24 injured in an explosion at a Coptic Christian church, possibly in retaliation for the rape of a Muslim girl.

Several years ago, I helped set up Save the Assyrians, to put pressure on Iraq to protect its minorities. The campaign had all-party support in Britain and a role in persuading the European Parliament to work with the Iraqi authorities to acknowledge the rights of Iraq's Christians. Britain has a special responsibility towards the Assyrians, who helped the British to police Iraq in the early years of the last century. Thousands were massacred in 1932 for "collaborating".

Now, Iraq's remaining Christians want an autonomous Christian province in their ancient Ninevah Plains homeland in northern Iraq. While Britain or the US may not help their cause, for obvious reasons, the UN, EU and Commonwealth could add their not inconsiderable weight. President Talabani of Iraq declared in November that he had "no objection to a Christian province in Iraq". One Assyrian exile in Britain, however, told me, "They keep talking, but nothing happens."

There is a widespread view among the Iraqi Christian diaspora that their government is simply allowing what some now see as an inevitable and unstoppable exodus of one of the world's most ancient civilisations.

Al-Qa'ida will have judged that a continuing campaign of terror could send Iraq's remaining Christians fleeing within a decade. The terrible irony is that the fate of Iraq's ancient Christian communities may have been sealed when the avowedly Christian leaders of Britain and America decided to topple Saddam Hussein.