Police deaths: The officers killed in the line of duty
The deaths of Fiona Bone, 32, and Nicola Hughes, 23, during a routine operation in Tameside, Greater Manchester, are the latest in a long list of UK police fatalities.
It is the first such incident to involve two female officers.
Speaking about their deaths, Greater Manchester Chief Constable Sir Peter Fahy said: "Day in, day out, police officers go about their duty, go into dangerous situations, unexpected situations, and show great bravery, great courage and are with people at the very worst moments in their lives.
"This is exactly what these two officers were doing."
According to the National Memorial Day organisation, more than 4,000 police officers have been killed in the line of duty since 1792, when the first salaried constables went on duty.
The organisation said 256, including PC Bone and PC Hughes, have been shot since 1945.
Joe Holness, who founded the organisation, said the figures rose each year by an average of between 10 and 15 officers.
The charity, Police Roll of Honour Trust, has named every UK officer who has been killed in the line of duty.
They include PC Sharon Beshenivsky who was shot dead during a bungled robbery in Morley Street, Bradford, on 18 November 2005.
Two men were jailed for life for her murder. She had been responding to an armed robbery call with fellow PC Teresa Milburn.
PC Beshenivsky had been a police officer for just nine months and became the first female officer in 20 years to be shot dead by a criminal in England and Wales.
In one of the most notorious incidents of all - almost five decades ago, in 1966 - armed robber Harry Roberts and an accomplice shot and killed three officers.
Det Con David Wombwell, Sgt Christopher Head and PC Geoffrey Fox died when they stopped a van in Shepherd's Bush, west London.
Other more recent high-profile deaths include PC Alison Armitage, 29, who died after being run over twice by the driver of a stolen car in the car park of a derelict pub in Oldham, in 2001.
Thomas Whaley, 19, denied her murder and was jailed for eight years after pleading guilty to manslaughter.
PC Ian Broadhurst was shot dead after stopping a motorist on Boxing Day in Leeds in 2003.
David Bieber, a former US marine, was jailed for life for murder after Newcastle Crown Court heard the 34-year-old officer was shot in the head at point blank range despite pleading for his life.
In 2003, PC Stephen Oake was repeatedly stabbed during an anti-terror raid in Manchester. He was holding on to Algerian national Kamel Bourgass during the operation, to stop him escaping.
Bourgass was sentenced to life in prison on 29 June 2004 for the murder of PC Oake and the attempted murder of three other Greater Manchester officers.
Northumbria Police officer PC Joe Carroll, 46, died following a crash on the A69 near Hexham in April 2006 as a prisoner tried to escape from his patrol car.
Army instructor Steven Graham, 39, was jailed for five-and-a-half years for his manslaughter.
In 2007, 37-year-old PC Gary Toms from the Metropolitan Police died after suffering serious head injuries while investigating a robbery.
Armed officers had attempted to stop a car in Stratford, east London and followed it to Ashlin Road in Newham.
In 2008, PC Ian Terry, 32, from Burnley died after being shot in the chest by a Greater Manchester Police colleague during a training exercise at a disused factory.
PC Terry's widow, Joanne, has said she is "bitterly disappointed" no-one will face criminal charges over his death.
Killed in training
Drug addict David Parfitt was found guilty of manslaughter after PC Ged Walker died after being dragged more than 100 yards trailing from a car door before suffering fatal injuries.
Parfitt, who was at the wheel of a stolen taxi when he dragged the officer at a speed of 30mph along a street in Bulwell, Nottingham, was jailed for 13 years.
Father-of-three Stephen Oake, a 40-year-old plain-clothed special branch officer, was stabbed in the chest and died soon afterwards in hospital in 2003.
Four others were injured during a counter terrorism operation in Manchester.
Officers had been arresting a man in a raid linked to the discovery of the deadly poison ricin in London when they were attacked after holding him for an hour.
Traffic officer PC Jon Odell, who lived in Margate, died in hospital after being dragged about 50 yards down a road while carrying out speed checks in December 2000.
His hat, shoes, belt, baton and CS spray were strewn along the path of the car, and driver Wayne Rule, 25, was jailed for nine years after admitting manslaughter.
In 2004, Det Con Michael Swindells, 44, died from a single stab wound in Birmingham.
Police admitted he had not been wearing a stab-proof vest when he was attacked on a canal tow path which heads under the M6 toward Spaghetti Junction.
Earlier this year PC Ian Dibell, 41, of Essex Police was killed while taking action, despite being off duty.
He had intervened in an incident close to his home in Clacton-on-Sea to protect a member of the public from an armed man. PC Dibell was shot dead.
PC David Rathband, who was found dead earlier this year, had been shot in the face and blinded by fugitive Raoul Moat in July 2010 while sitting in his patrol car in Newcastle.