Police have launched a fresh investigation into the murder of a North-East police constable killed in riots in north London more than 18 years ago, Scotland Yard said today.

Pc Keith Blakelock was hacked to death during the Broadwater Farm riots in Tottenham in October 1985.

Winston Silcott was convicted in 1987 of his murder but the conviction was later overturned on appeal because of ''unsafe'' police evidence.

Scotland Yard today said the Metropolitan Police Specialist Crime Directorate had now launched a fresh investigation into the murder of Sunderland-born PC Blakelock.

A fresh investigation had also been launched into the attempted murder of Pc Richard Coombes, who was injured in the riots, a spokeswoman for Scotland Yard said.

The cases relating to both men had been under lengthy review since January 2000.

Police said the review had identified ''new lines of inquiry'' which were to form the basis of the new investigation.

The investigation is being led by Detective Superintendent John Sweeney from the Specialist Crime Directorate, overseen by Commander Andy Baker.

Assistant Commissioner Tarique Ghaffur, head of the Specialist Crime Directorate, said it would be a ''tightly focused inquiry''.

''The review took us in a number of directions, but there are some specific leads which we are now looking at,'' he said.

''We cannot go into exact details as this could seriously prejudice our inquiries. Suffice to say that we believe this information is strong enough to merit further investigation.

''At this stage we cannot be sure where the new inquiry will lead us but I would like to reassure members of the local community that this is not something we have undertaken lightly.''

Det Supt Ghaffur said the investigation would be focused on the cases of PC Blakelock and PC Coombes, not on ''any other incidents'' which happened during the riots.

PC Blakelock, who left three sons, Mark, Kevin and Lee, was set upon by a mob and hacked to death during the riots on October 6, 1985.

The riots were sparked by the death of 49-year-old Cynthia Jarrett during a police raid.

She collapsed and died of heart failure after four policemen burst into her home.

Winston Silcott, Mark Braithwaite and Engin Raghip were all originally found guilty of PC Blakelock's murder, but the Court of Appeal overturned their convictions in 1991.

Less than two months ago, PC Blakelock's widow said she still believed his killer would be brought to justice.

Mrs Elizabeth Johnson, 52, who now lives in Boldon Colliery, on the outskirts of Sunderland, has since remarried.

She spoke after it emerged that Silcott was to be freed from jail after serving a sentence for the murder of another man.

Mrs Johnson said at the time: ''If the man is going to be let out of prison then nothing I can do will change that.

''But I do believe that guilty people will be brought to justice and we will see people being punished for what they did to Keith.

''They have deprived a family of a father and a grandfather.

''It never leaves you and it never goes away, you just get better at handling the situation.

''People cannot get away with murdering people and I believe that life should mean life.

''If they don't get what they deserve in this life they will in another.''

''I must stress that the investigation is wholly focused on the murder of Keith Blakelock and attempted murder of Richard Coombes and not on any other incidents which occurred during the Broadwater Farm disturbances,'' he said.

The decision to reopen the case had been taken only after consultation with PC Blakelock's family, he added.

''We accept that the decision to launch this fresh investigation will be difficult for some sections of the community,'' he said. ''MPS officers have consulted with a wide range of people in reaching this decision to reopen the case.

''We are grateful for the input we have received, particularly from Keith's family, the local community and the wider black community, whose views have greatly shaped how we will approach this new inquiry.

''This was a brutal murder on an unarmed police officer who was protecting the local community.

''This new inquiry is a mark of our commitment to protect our police officers and the people they serve and bring to justice those who seek to harm them.''