Scotland’s chief law officer Frank Mulholland QC has announced that he will step down following May’s election.

Lord Advocate Mr Mulholland told First Minister Nicola Sturgeon that he was leaving the post after five years.

During his time in the role, Mr Mulholland, 56, has handled some of the most challenging and high profile cases in the country.

This included the re-trial of Angus Sinclair for the notorious World’s End murders in Edinburgh, after the double jeopardy law was amended.

Mr Mulholland had been involved in the original World’s End trial which dramatically collapsed due to "insufficient evidence" in 2007.

Other key cases overseen by Mr Mulholland include the Lockerbie bombing, the disappearance and murder of Moira Anderson in 1957, and murder cases including the so-called "limbs in the loch" killing of Barry Wallace, the re-trial of Nat Fraser over the death of his wife Arlene, and the murder of Vicky Hamilton by Peter Tobin.

He also supervised an inquiry into the investigation of the murder of Andrew Forsyth in Dunfermline in 1996.

Mr Mulholland rose through the ranks after joining the Crown Office and Procurator Fiscal Service as a depute in Greenock in 1984.

During his time as Senior Advocate Depute he successfully prosecuted the case against Transco in 2005 following the Larkhall gas explosion in 1999. As a result the company was fined £15m - a record fine for the UK.

As Lord Advocate he vowed to tackle unresolved homicides in Scotland by setting up the Cold Case Unit.

He also set up dedicated specialist units including the National Sexual Crimes Unit, the Wildlife and Environmental Crime Unit and the Health and Safety Unit.

He said on Wednesday: "It has been a real privilege to serve as Lord Advocate, leading Scotland’s prosecution service and providing independent legal advice to the Scottish Government.

"However, after nine years as a Law Officer – the last five as Lord Advocate - I have decided it is the right time to step down and do other things."

He said the Crown’s expertise in handling offences such as rape, domestic abuse, serious organised crime, counter terrorism and cold cases had helped Scotland become "one of the most effective prosecution services in the world".

Mr Mulholland said improvements to the service had also given victims greater confidence to report crimes.

He added: "It’s been an honour to do this job working with so many dedicated and talented people to deliver justice in some of the most demanding and challenging of cases."

Ms Sturgeon paid tribute to Mr Mulholland following the announcement on Wednesday morning.

She said: "Frank Mulholland has fulfilled the challenging roles as head of Scotland’s prosecution service and as the Scottish Government’s principal legal adviser, with dedication, energy, integrity and intellect.

"He has played a central role in many innovations to our justice system, including leading Scotland’s first successful ‘double jeopardy’ murder prosecution and agreeing a historic communiqué with the heads of prosecution services from across the UK and Ireland to work together to tackle the cross-border crime of human trafficking.

"He has introduced a number of significant reforms to the Crown Office."

The First Minister added: "The final Bill considered by the Scottish Parliament in the current session includes provisions for a new specific offence against the unauthorised sharing of intimate images, an issue which he played an essential role in highlighting."

Mr Mulholland will formally step down when his successor is appointed by the Queen.