It is impossible to know whether the claim of responsibility made on the internet is genuine, unless the group offers some proof that it was responsible.

Far more telling are the facts emerging from the bombings, which have all the hallmarks of an al-Qaeda attack and are similar to the deadly rush-hour bombings by al-Qaeda terrorists against commuters in Madrid last year.

First the bombs were synchronised to go off at the same time and cause maximum terror. The same method was used by al-Qaeda against the US embassies in Nairobi and Dar es-Salaam, against the World Trade Centre in New York and the Pentagon in Washington on September 11, 2001, against nightclubs frequented by Australian tourists in Bali, and against the British consulate-general and the HSBC bank in Istanbul.

Unlike previous terrorist attacks by Irish republican groups, al-Qaeda never issues a warning and the intent is to kill and injure as many people as possible, as happened to the rush hour commuters today.

The other circumstantial evidence is that British intelligence has been expecting an attack on London for the past four years.

Tony Blair has been George Bush's closest ally in his War on Terror, sending British troops to Afghanistan and Iraq and collaborating closely with pro-Western Arab regimes, whose overthrow al-Qaeda is committed to.

More than one senior intelligence source has told me in the past that "it is not a question of whether but when". They claim to have thwarted past attempts at exactly this sort of attack.

If today's claim is genuine it raises a disturbing new twist to the campaign of violence. The group specifically linked the operation to Iraq and warned Italy and Demark to pull their forces out or face the same threat of terror.

Previously, it was assumed that Britain was on al-Qaeda's hit list for all the reasons I mentioned above. But the original al-Qaeda group headed by Osama bin Laden is much weakened by the overthrow of the Taleban regime in Afghanistan and the killing or capture of its followers.

The real power now resides in Iraq with Abu Musab al-Zarqawi, the Jordanian terrorist leader responsible for the worst acts of violence by the insurgency in Iraq over the past two years.

His group is supported by hundreds, possibly thousands of mujahidin volunteers from across the Arabic and Islamic world, including British citizens. His followers are well trained and ruthless. If they have decided to launch a campaign in Britain it could be the start of a long and bloody conflict.

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