A policeman was killed and four wounded while raiding a house in connection with the recent recovery of the deadly poison ricin. Three terrorist suspects were arrested as a result of the raid.

Police said the raid in the northern city of Manchester was part of an ongoing operation by anti-terrorist officers. British authorities have been warning citizens for months that the country faces a serious threat of terrorist attack.

Once inside the residence, officers found three men of North African origin. Chief Constable Michael Todd said the officers had been searching the apartment for about an hour when "one of the suspects managed to get free, get a knife, and stab a number of our officers." One 40-year-old officer died from a wound to the chest. Another was seriously injured with a stab wound to the chest and three received minor injuries, including stab wounds and a broken ankle.

The three suspects, arrested under counterterrorism legislation, were jailed and questioned by anti-terrorism officers.

Four other men, also described as of North African origin, appeared in a London court Monday charged with chemical weapons and terrorism offenses in the ricin case. Anti-terrorist police earlier this month uncovered traces of the powerful poison in an apartment in north London's Wood Green district.

Police have dropped terrorism allegations against six people arrested earlier in the week in the southern England town of Bournemouth. Police denied reports linking the men to an alleged plot to use ricin. Police were investigating whether the suspects had participated in a terrorism-related hoax and broken immigration laws. They said this was unrelated to the discovery of ricin in London.