JACK McConnell said yesterday he was "obviously disappointed" to receive the resignation of Lord Boyd, who could be "unashamedly proud" of his legacy.

The First Minister said the Lord Advocate's legacy included modernising the criminal justice system, court reform and the successful conduct of the Lockerbie trial.

"The lives of people living in Scotland have been improved as a result of your personal dedication to justice at every level, " he said.

But the sentiments were not echoed by opposition MSPs and many lawyers last night.

To his critics, Colin Boyd, QC, ennobled earlier this year as Lord Boyd of Duncansby, will be remembered for the creeping politicisation of the Crown and Procurators-Fiscal Office, the bungled Shirley McKie affair, and, potentially, mistakes in the Lockerbie trial.

More supportive voices said he was a steady, if unflamboyant, officeholder who should be thanked for the smooth operation of justice since 1999.

Born in Falkirk in 1953, Lord Boyd was educated at Wick High School and George Watson's College in Edinburgh. He graduated from Manchester University with a BA in politics and economics, and from Edinburgh with an LLB.

Called to the Scottish Bar in 1983, he took silk in 1995, specialising in administrative law. After Labour was elected, he was appointed Solicitor General for Scotland, taking over from Lord Hardie in February 2000.

As Lord Advocate, he was in charge of the biggest trial in Scottish legal history: Lockerbie. He is proud of the prosecution of Abdelbasset al Megrahi at Camp Zeist in the Netherlands, which he said in his resignation letter "showed Scottish justice at its best".

While conspiracy theorists suggest he is departing as that trial looks increasingly likely to be granted a fresh appeal, it is thought the details of any new appeal are more likely to highlight failings by members of the police and security agencies.

More recently, he has restructured the Crown Office, reformed the appointment of advocates depute and beefed up victim support.

But the departure came 24 hours after Lord McLuskey, the retired High Court judge, published a stinging attack, accusing him of presiding over "confusion, injustice, mystery and rumour" in the Shirley McKie case and of allowing error-strewn legislation on law reforms to go before Holyrood. "The Lord Advocate has got uncomfortably close to the politicians, " he wrote.

Alex Neil, SNP MSP, said Lord Boyd's departure was "inevitable" after the McKie affair, adding: "His tenure has proven his incapability to deal with the role of Lord Advocate."

Tory Margaret Mitchell said his exit was "astonishing" when unanswered questions remained over his actions.

One QC said he was "universally unpopular" in the Faculty of Advocates, seen as plodding, uninspired and a wrecker of the Crown Office.

However, Derek Ogg, QC, described Colin Boyd as "the quiet man" and the "brains behind the Scotland Act".

Elish Angiolini, Solicitor General, is favourite to be the next Lord Advocate. Other contenders, and possible replacements for Ms Angiolini, include Lord Davidson of Glen Clova, the Advocate General for Scotland, Paul McBride, QC, John Beckett, QC, principal advocate depute, and Dorothy Bain, assistant principal advocate depute.

THE ROLE

As head of the Crown Office and Procurator-Fiscal Service, the Lord Advocate is in charge of the systems of criminal prosecution and the investigation of deaths in Scotland and is the country's most senior law officer.

The role evolved from that of the king's advocate in the 1400s.

Various advocates spoke for the king in court, until by 1494 a single office holder represented the king in treason trials and in civil litigation.

It was not until 1579 that the post became chief public prosecutor in criminal cases.

The 1998 Scotland Act protects the Lord Advocate's work from interference by ministers or the Scottish Parliament.

However, the Lord Advocate also acts as the chief legal adviser to the Scottish Executive and is a member of the cabinet.

This dual role has led to criticism that the Lord Advocate cannot be truly independent of ministers.

There have been repeated calls for reforming the post.

His salary is the same as that of a minister - GBP102,474 last year.

The Lord Advocate is also one of the great officers of state of Scotland, charged with maintaining and protecting the Honours of Scotland.