The conviction of an Algerian terrorist for the murder of Detective Constable Stephen Oake and the acceptance by the jury that there was a plot to manufacture the deadly poison ricin and use it in household products to cause panic raises extremely serious questions. The implications for British Intelligence, national security and the immigration service are deeply disturbing and ought, in the midst of an election campaign, to underline the urgent need for reform.

The trial of Kamel Bourgass, a shadowy figure whose true identity may still be unclear, has been held in camera, so no details have emerged till now of the chain of coincidences, intelligence trails and missed clues that led to the chance discovery three years ago of a terrorist operating from a flat in North London. The case was complicated by the presence of a group of Algerians who have been cleared of terrorism charges, but, apart from two, are linked to immigration abuse. What is clear, however, is that the plot to cause random casualties by smearing car door handles and toothbrushes on sale in chemists with ricin was real, was linked to al-Qaeda’s assault on Western society and came dangerously close to success. This case is the most telling evidence yet that Britain has been the target of extremist terrorism and that official warnings should not be regarded as a political plot.

While the police have emerged from the case with an enhanced reputation for their pursuit of evidence in the murky world of extremists infiltrating this country, no such credit can go to British Intelligence. For years, the French warned MI5 that Algerians implicated in atrocities within Algeria and abroad were slipping into Britain and claiming asylum. The British authorities took little notice, insisting that they had no evidence of wrong-doing. This negligence changed after the September 11 atrocities in 2001 and in the wake of new anti-terrorist legislation: co-operation with France was stepped up, and a closer watch was placed on Algerian extremists.

Nevertheless, until an Algerian tip-off, Bourgass and others were able to slip in and out of Britain repeatedly since 2001, using an array of false identities. This makes a mockery of the supposedly increased surveillance. More shockingly, however, it reveals appalling laxity by the immigration service. Accomplices were not stopped at ports and airports, nor were the suspicions that a mosque was being used as a centre for the fraudulent provision of false papers and identities properly investigated.

There are some 100 million entries to Britain each year. Only a tiny proportion are by illegal immigrants or those intent on terrorism. But the manpower available to deal with the threat they pose is pitiful; after hours, there are no immigration enforcement officers on duty in London at all. The ricin plot should prompt swift and far-reaching changes. However bizarre, it could have created panic and would inevitably have increased tension in relations with and within Islamic communities. Terrorism is an enduring danger that must be properly confronted.