The discovery of the lethal poison ricin in a London apartment last week was the most recent incident in an anti-terror campaign that British and French security sources say is increasingly focused on Algerian immigrants. Many Algerian extremists are reportedly moving from France to Britain as French security services crack down on their activities in France.

The Algerian connection to terrorism is rooted in a ten year conflict in the North African country between the government there and Islamic radicals. The conflict has led to the displacement of many of the radicals to France, Algeria's former colonial ruler.

In France, they blend in easily with Europe's biggest Muslim community.

But after a series of terrorist attacks in France conducted by Algerian extremists, most notably the hijacking of an Air France jetliner in 1995, the French security services have clamped down on known extremists and, in many cases, penetrated their organizations.

Experts on intelligence matters, like Jean Francois Daguzan, of the Foundation for Strategic Studies in Paris, say that as life became more difficult over the past few years for Algerian terrorists in France, they found refuge in Britain. Mr. Daguzan said that, for a long while, British intelligence was either uninterested in, or unaware of the potential danger represented by these men.

"Using Britain like a safe haven, they organized and structured their community in order to provide support to the French terrorist community and so the links are very close," he said.

But Mr. Daguzan says that since the September 11, 2001, terrorist attacks in the United States, Britain has begun to take the threat of North African extremists on its territory more seriously and that British and French intelligence agents have been working much more closely.

Security sources in Britain and France say the arrests of seven North Africans in London last week and the arrest of six more people Sunday in the coastal town of Bournemouth, are the fruits of that cooperation. They say the arrest before Christmas near Paris of a group of Chechen-trained terrorist suspects who are of Algerian origin was the breakthrough that resulted in British authorities finding ricin in the London apartment.

Although British police do not identify terrorism suspects by nationality when they are arrested, a British security official says most of the people arrested so far in the ricin case are of Algerian nationality or origin. He says that, of the 200 or so important arrests made in Britain and elsewhere in Europe on terror charges since September 11, the vast majority involved North Africans and Algerians in particular.