For two years recruiters have been providing fake documents, training and finance for the fighters, communicating secretly over the internet and liaising with cells based in Syria.

Despite much closer links between intelligence and law enforcement agencies across Europe, experts say that the terrorists are moving between European countries to avoid detection.

“The terrorists are operating internationally, even if the law enforcement agencies are a long way behind them,” said M J Gohel, chief executive of the Asia-Pacific Foundation, which monitors terror organisations. “We are seeing another Al-Qaeda-type organisation in the making.”

Of 16 people arrested in Spain last week, 11 are said to be linked to the Iraqi branch of Al-Qaeda run by Abu Musab al-Zarqawi. The other five are alleged to be members of the group behind the March 2004 Madrid train bombings.

These five, in turn, are closely linked to Mohamed Achraf, an Algerian awaiting extradition from Switzerland. Achraf is known to have been planning a lorry bomb attack on the National Court in Madrid. The aim was to punish Spain, kill the investigating judges and destroy terrorism records in a single action.

Telephone records prove a link between Achraf and Mohammed Bouyeri, the alleged killer of Theo van Gogh, the Dutch film maker.

Other members of this group helped suspects in the Madrid bombing to flee the country. Mohamed Afalah, one of those who escaped, is thought to have blown himself up in a suicide attack in Baghdad on May 19, killing an Iraqi soldier and wounding eight others.

Three other suspects arrested in Germany last week were also connected to Iraq. More than 150 officers raided two dozen addresses searching for evidence that Ansar al-Islam, the Kurdish terrorist group, was using central Europe as a fundraising and smuggling base for its operations in Iraq.

It was the fourth such sweep since December, when three men were arrested on suspicion of plotting to kill Ayad Allawi, the interim prime minister of Iraq, while he was visiting Germany.

Amin Lokman Mohammed, another suspect, is on trial in Germany, charged with organising trips to Iraq for at least a dozen possible suicide bombers.

In France 10 people were arrested earlier this year on suspicion of being part of an Islamist cell recruiting fighters for Iraq.

At least three French nationals have died in Iraq fighting coalition forces. Another five have been identified as still fighting; one is believed to command a 20-strong insurgent cell.

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