MI5 and immigration investigators were ordered to carry out an urgent check on all Algerian arrivals after the discovery of a terrorist cell manufacturing the poison ricin in a London flat.

So far they can find only a fraction of those they want to question. More than 9,000 Algerians are still in Britain years after having their asylum claims refused.

Officials were told to authenticate their true identities and to re-examine the claims they made when they first arrived here. Investigators have discovered that asylum applications made by some of the Algerians rounded up in recent raids bore “an uncanny similarity” in the personal details they gave. The suspicion is they were schooled in what to say to convince immigration staff to allow them to remain in Britain.

The investigators began closely examining the applications of other recent Algerian asylum-seekers and discovered that some of those had also given similar accounts when they entered the country.

Some claimed to have been arrested in the same raid by Algerian Government forces, tortured in the same prisons, on the same dates, and had lost the same numbers of family members, all from the same remote areas.

One official close to the case said: “It is a nightmare at the moment as we can’t find most of these people we want to talk to, so we can establish if they are who they say they are. The fear is obviously that some of those we cannot account for may have links with militant groups already operating here.”

Police are still trying to check the identities and backgrounds of some of the 18 Algerians arrested in Edinburgh, Leicester, Bournemouth, Manchester and London. Of those, 15 are believed to be asylum-seekers.

The priority for investigators is to trace these missing Algerian asylum-seekers who have probably moved to new addresses in Britain, possibly using different identities.

Official figures say that around 10,000 arrived here in the past decade, but unofficial estimates by community leaders put the real number closer to 40,000.

Some carry French passports, or have dual nationality, and others are here on bogus identity documents which some cells are expert in manufacturing.

The sudden focus on Algerian extremists has exasperated French investigators, like Judge Jean-Louis Brugière, who say they have been warning Britain for years of the risks that militant groups pose.

Some of those arrested in Britain have been expelled from France. At least two suspects being held in Britain have previously been in French custody but were still allowed into Britain as asylum-seekers.

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