The shootings occurred after police were informed that a flat in Cato Road, Clapham, south London, was being ransacked.
A local constable sent to investigate heard three gunshots in the same road coming from a house which had previously been under observation for drug dealing. When he went to investigate he was shot down.
The constable was taken to St Thomas's hospital in Lambeth where he was certified dead at 9.50pm of a single gunshot wound. He also had chest injuries.
Police also found a man fatally wounded inside the flat, which police suspected of being used by drugs dealers, said Det Supt John Jones, who is charge of the police operation in Clapham. He had been shot several times in the abdomen.
After the shootings three men were seen running away towards Clapham High Street. One was carrying a gun. They escaped from pursuing officers.
A resident of Aristotle Road, which runs next to Cato Road, said: 'It all started around 9pm with a huge wailing of police sirens and a helicopter hovering around.
'One was aware of some loud noises - though whether it was shots or a car backfiring was impossible to say.
'The high street is closed off now. There are no cars moving up and down at all and just one or two pedestrians, but there are a hell of a lot of police.'
Mike Bennett, chairman of the Metropolitan Police Federation, said the killing of another policeman on duty would increase calls from the ranks for officers to be armed. 'The only protection against the gun is the gun.'
He called on the Government to ensure police killers spent the rest of their lives in jail.Reuse content