They were found to be in possession of chemical materials and a special nuclear, biological and chemical warfare protection suit. French authorities said that they had links with terrorist suspects being detained in Britain.

The police raid came as the British Government was given warning of increasing intelligence information that alQaeda terrorists are plotting a chemical or biological attack in Britain.

Evidence has been uncovered that al-Qaeda terrorists have entered Britain using false identities, which has made it difficult to track their movements across Europe.

Information about visits to this country by al-Qaeda figures has emerged from agents working for the police and MI5, but has been provided too late to detain the suspects before they have left Britain.

Earlier this week the Metropolitan Police warned the public to be extra-vigilant because of the potential terrorist threat over the Christmas period.

After the arrests in a flat on the outskirts of Paris, Nicolas Sarkozy, the French Interior Minister, said that those arrested had links to terrorist suspects detained in Britain.He told the French parliament that three of the four arrested in France “seemed to have been in Chechnya”.

Officials said two flasks containing a powder and a liquid substance were found in the suspects’ flat in Courneuve, a suburb north of Paris.

Military scientists with specialist knowledge of germ warfare were last night analysing them to find out whether they could have been used to make chemical weapons.

French radio said the label on one of the flasks revealed that it contained iron perchloride, a corrosive chemical that gives off dangerous fumes when combined with mercury.

However, experts at the Bouchet military laboratory, in the Essone department outside Paris, said that further tests were needed to confirm the substance as iron perchloride.

Detectives also discovered a suit designed to give chemical, biological and nuclear protection in the flat in Courneuve, along with chemical formulae, documents, a computer and $5000 and €20,000 in cash.

The discoveries have underlined for Britain’s security and intelligence services the growing dangers facing this country. The security threat level for Britain is already at “high”, which is only one level down from “imminent” on the list of six terrorist-alert gradings. The Cabinet Office’s Civil Contingencies Secretariat, which would help to mastermind the nation’s response to a terrorist emergency, is already on heightened alert for a possible attack in the next few months.

One of those detained in Courneuve was named by police as Mirouane Ben Ahmed, a 29-year-old Franco-Algerian. Detectives said that he was connected to the so-called Frankfurt cell that was dismantled in a trans-European police operation in December 2000 as it was allegedly preparing to carry out an attack on a Christmas market in Strasbourg, eastern France.

The other three suspects arrested in Courneuve were a Franco-Algerian, a Franco-Moroccan, as well as M Ben Ahmed’s wife.

The raid on Courneuve was carried out by officers working for the French intelligence agency, La Direction de Surveillance du Territoire, under the orders of Jean-Louis Bruguière, the anti-terrorist magistrate.

He opened an inquiry into the Courneuve suspects on November 13 after intelligence agents reported a high level of activity among al-Qaeda members linked to the Frankfurt group.

Detectives in Paris also said the three suspects arrested in Courneuve had links to Abu Doha, a London-based Algerian who was named in a German courtroom this year in connection with al-Qaeda.

Last night it was revealed that a suspected al-Qaeda terrorist wanted in America over the bombing of the US Embassies in Kenya and Tanzania has been treated for cancer at an NHS hospital in the Home Counties.

Ibrahim Eidarous, who is at Broadmoor high security hospital while his legal team fights extradition proceedings, was taken under guard to the oncology unit at Wexham Park Hospital in Slough, Berkshire. Doctors there removed bone marrow to test for cancer.

A Broadmoor source said: “Anybody detained in an NHS hospital is going to get appropriate treatment from the NHS as and when required.”

A Home Office source ridiculed suggestions in the tabloid press that Mr Eidarous may have jumped NHS waiting lists.

Mr Eidarous, 40, of West London, is accused of being a member of Egyptian Islamic Jihad, a terrorist organisation that merged with al-Qaeda in 1997. He is also accused of conspiring to murder US citizens abroad and of giving direct orders to the cell that carried out the bombings in Nairobi and Dar es Salaam 1998 that killed 224. He denies the charges.

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