PC Keith Blakelock was almost decapitated by a mob shouting "kill the pig" during the Broadwater Farm riots in north London in 1985, a court has heard.

Nicholas Jacobs, 45, is on trial accused of being part of the gang that stabbed the then-40-year-old officer more than 40 times during the chaos at the housing estate in Tottenham.

Jacobs, who was 16 at the time of Blakelock's murder, was arrested in 2010 after Scotland Yard reopened investigations into the father-of-three's murder. He was charged in July.

Opening the prosecution at the Old Bailey, Richard Whittam QC said the riots at Broadwater farm had a more "sinister edge" than the disorder which occurred during the London riots of 2011.

The violence erupted following the death of Cynthia Jarrett, who collapsed and died of a heart attack as police searched her home looking for stolen goods following the arrest of her son.

Blakelock was killed as he tried to protect firefighters tackling a blaze in a supermarket. The court heard how he was among a number of officers who were then set upon by the armed mob.

Whittam said: "Very heavily outnumbered and fearful they may become trapped, both the police and the firefighters were forced to retreat. Outside the flats, as they ran for safety, PC Blakelock and PC Richard Coombes went to ground and were set upon to shouts of 'kill the pig' and the like.

"PC Coombes was very fortunate to survive. PC Blakelock did not. The attack on him was without mercy. In the ferocious attack his helmet came off.

"He was beaten and stabbed to death before his colleagues were able to force the attackers away.

"PC Blakelock suffered something in excess of 40 stab-type injuries and there appears to have been an attempt made to decapitate him."

Whittam told the jury they must decide if Jacobs was part of the group who attacked the officer "without mercy" that day.

Winston Silcott, Mark Braithwaite and Engin Raghip were convicted of Blakelock's murder in March 1987, but all three convictions were quashed four and a half years later.

Jacobs denies murder.

The trial continues.