Small groups of chemicals experts have been detected in several European countries and have developed ways of communicating with each other that allowed them to avoid being exposed.

"We have underestimated the terrorists' willingness and capacity to develop chemical weapons," the French official told the Financial Times. He said a recent wave of arrests in Britain and France has revealed how far they had developed their plans.

The groups appear to operate separately from other cells planning attacks using ordinary explosives. Several of them are believed to have links to Islamic militants in the breakaway Russian republic of Chechnya. Western intelligence services allege that extremists linked to al-Qaeda have carried out experiments in chemical warfare in Chechnya.

In January French anti-terrorist police arrested five people in the Lyons suburb of Venisseux - three of them from the same family - on suspicion of involvement in planning terrorist attacks. Nicolas Sarkozy, then interior minister, said that one of the detainees, Menad Benchallali, "was trained to produce chemical substances".

Two of the detainees admitted a plan had been devised to attack Russian targets in France using ricin poison and botulinum bacteria. French officials say Mr Benchellali received chemical weapons training in Georgia's Pankisi Gorge, a haven for Chechen fighters.

On April 6 British anti- terrorist officers uncovered a possible plot to use osmium tetroxide in an attack. The chemical can cause death or blindness if dispersed in an explosion. UK security officials have refused to comment on the alleged plan, though US officials said it was in its early stages.

The alleged plotters were reported to have been in direct contact with extremists in Pakistan, as the plot was discovered when their telephone calls were monitored by GCHQ, the UK government's electronic surveillance centre.

"The Pakistani element [in developing these weapons] was also totally underestimated, as was the experience developed in Chechnya," said the French official. He added that militants within the Pakistani Islamist group Lashkar-i-Toiba, which has close links to al-Qaeda, had helped develop chemical weapons skills now dispersed to sever al parts of the al-Qaeda network.

"The thing that is most clear is that the people with the knowledge of chemicals are very organised," the French official said. "There are links between the groups that have chemical expertise. These groups are not present everywhere, though Chechnya is where they learned this skill."

The arrests in January in Venisseux led to the discovery of vital clues about the links between alleged extremists with knowledge of chemicals and experts trained in Chechnya and Afghanistan.

"The group arrested in Venisseux has links to Chechnya, but also to Abu Musab al-Zarkawi," the official said.

Al-Zarkawi, a Jordanian thought to be in Iraq, is said by intelligence officals to have run classes in chemical warfare at an al-Qaeda training camp in the Afghan city of Herat in 2000-01. A taped statement attributed to al-Zarkawi was broadcast on an Islamist website this week, in which he said that Iraq's Sunni Muslims should "burn the earth under the [foreign] occupiers' feet" in Iraq.