A policeman was stabbed to death and four others were wounded as they raided a house and arrested three terrorist suspects Tuesday in an operation linked to the recent discovery of the deadly poison ricin.

Police said the raid in the northern city of Manchester was part of an ongoing operation by anti-terrorist officers and was connected with the ricin find in a London apartment on Jan. 5. British authorities have been warning for months that the country faces a serious threat of terrorist attack.

Assistant Chief Constable Alan Green said there was no sign of the deadly poison at the site of the raid. Ricin has been linked to the Al Qaeda terror network and Iraq.

Prime Minister Tony Blair said in a statement he was "shocked and very saddened" by the death of the officer. "His death and the injuries to the other officers involved in this incident underline the dangers that our police and security forces face in these times."

The National Police Federation said that 55 were killed in the line of duty since 1980. Usually officers do not carry firearms.

Green told reporters that a team of officers and immigration officials intended to arrest a man under counterterrorism legislation.

Once inside the residence, they found three men of North African origin. Chief Constable Michael Todd said 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 in hospital 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 officers' names were not released.

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

The British government has issued several general warnings that Britain could be the target of terrorist attacks in the wake of the Sept. 11 attacks in the United States. Blair told Parliament last month that "barely a day goes by" without some new piece of intelligence warning of threats to British interests.

In November the government issued -- and then hurriedly withdrew -- a statement warning that Al Qaeda might be prepared to use a radiological device known as a "dirty bomb," or some kind of poison gas. It was replaced with a more general warning of terrorist threats.

Meanwhile, 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.

Earlier Tuesday, police dropped terrorism allegations against six people arrested Monday in the southern England town of Bournemouth.

Police denied news media reports linking the six to an alleged plot to use the deadly poison ricin.

Instead, police were investigating whether the six suspects, had participated in a terrorism-related hoax and broken immigration laws. Police declined to specify the nature of the possible hoax but said it was unrelated to the London ricin find.