A police officer was stabbed to death and four others were injured last night during the arrest of three terrorist suspects in an operation linked to last week's discovery of the deadly poison ricin.

The officer died during a raid on a flat in Crumpsall, north Manchester.

Greater Manchester Assistant Chief Constable Alan Green said the 40-year-old plain clothes constable died of stab wounds in hospital after receiving emergency treatment at the scene.

When the officers had arrived at the flat to detain a man under terrorist legislation, the suspect was with two other males.

He said: "It appears one of the men attacked the officer with a weapon - believed to be a large kitchen knife."

Police said as well as the fatally injured detective constable, four other officers, including a detective inspector, a sergeant and two constables, were injured. Three were stabbed and another was believed to have a broken ankle.

Mr Green said: "I am unable to give details of the counter-terrorist operation but I can confirm that it is connected to those led by the Metropolitan Police in recent days.

"We have sealed the premises and clearly public safety is paramount but there is nothing to indicate anything of that nature is there."

Prime Minister Tony Blair said: "I was shocked and very saddened to learn of the death of this brave police officer.

"It is an appalling tragedy and wicked in the extreme."

Shadow home secretary Oliver Letwin said: "The whole nation will mourn the passing of a hero who died defending us from terrorism."

Norman Brennan, director of Victims of Crime Trust and a serving officer for 25 years, said: "Britain is facing one of its blackest hours as far as law and order is concerned.

"With violent and general crime out of control and the criminal justice system in crisis and now the murder of a serving police officer, who I would suggest was unarmed, this Government needs to completely overhaul our criminal justice system."

Home Secretary David Blunkett said: "I commend the bravery and commitment of these police officers in defending us not only against dangerous criminals but against those threatening the safety of our country."

Jan Berry, chair of the National Police Federation, said: "Yet again, an officer has given his life. Our thoughts are with his family and colleagues at this time."

Simon Hughes, Liberal Democrat home affairs spokes-man, said: "The best response is sorrow and anger but followed by determination and renewed commitment to the rule of law."

The discovery of ricin in a north London flat early this month led to a nationwide alert and a number of arrests under the Terrorism Act 2000.

Since the September 11 attacks and the increasing tensions with Iraq, the question, in some minds, is not if Britain will be targeted but when.

Last night's fatal stabbing became the latest tragedy to join a notorious and increasing list of deaths in the line of duty. More than 30 officers have been killed in the past 30 years as they battle against increasing violence on Britain's streets.

Among the most infamous police murders was that of PC Keith Blakelock, from Sunderland, who was hacked to death during the Broadwater Farm riots in London in 1985.