Eight British citizens of Pakistani descent were being held under the Terrorism Act last night after police seized the ingredients of a potentially major bombing mission in Britain, writes Frank Millar, London Editor

MI5, MI6 and the Metropolitan Police anti-terrorist branch appeared to have prevented a planned atrocity when they discovered a massive half-tonne of chemical fertiliser – a common component in the Bali and Istanbul bombings – during a series of dawn raids on 24 premises across London and the Home Counties yesterday morning.

Police said the arrests and discovery – following weeks of surveillance – were not related to Irish republican terrorism or linked to the Madrid bombings.

Some 700 officers from five forces were involved in the simultaneous move on commercial and residential addresses in what police later described as Britain\’s most significant anti-terrorist operation for years.

The men – all but one of whom are understood to be in their late teens or early 20s – were being held at a high-security police station on suspicion of being involved in the commission, preparation or instigation of acts of terrorism.

Two were arrested in Uxbridge, west London, three in Crawley, West Sussex, and one each in Ilford, in east London, Slough in Berkshire and Horley in Surrey.

Surrey police said an 18-year-old man had been arrested at a Holiday Inn hotel near Gatwick Airport, although Scotland Yard refused to say whether an airport could have been an intended target.

Scotland Yard\’s deputy assistant commissioner, Mr Peter Clarke, confirmed that a key issue in the ongoing investigation concerned the purchase, storage and intended use of the half-tonne of ammonium nitrate fertiliser seized from a lock-up in Boston Road, Hanwell, which, like Uxbridge and Slough, is close to London\’s Heathrow Airport.

Ammonium nitrate – a key bomb-making ingredient – was used in the attack on Bali in 2002 and in the attack on British interests in Istanbul last year. It is also believed to have been used in an al-Qaeda attack on the US embassy in Nairobi in 1998 and in the Oklahoma bombing in 1995.

While police offered no information about the political affiliations of the suspects, speculation immediately turned on the possibility that an al-Qaeda-supporting cell had been planning a \”spectacular\” attack on a British target, similar to the IRA bomb at Canary Wharf.

However the spotlight was also turning on a group of Islamic extremists, part of the \”al-Tauhid group\”, first identified as being based in London and Luton – where police also carried out raids yesterday – in German intelligence documents last year. Al-Tauhid, while independent of Osama bin Laden\’s al-Qaeda organisation, follows a similar agenda and previously plotted to use poison in attacks in Britain and elsewhere.

Deputy Assistant Commissioner Mr Clarke yesterday stressed police awareness that \”the overwhelming majority\” of British Muslims were law abiding and rejected violence. However, Massoud Shadjareh, chairman of the Islamic Human Rights Commission, claimed the Muslim community was being \”demonised\” as a result of such raids. He added: \”These raids are usually given a lot of importance when they are taking place, but when people are released without charge it is not news.\”

The Home Secretary, Mr David Blunkett, said the finds were a \”timely reminder\” that British interests at home and abroad remained a target.