Police have been ordered to re-investigate the 2005 murder of Emma Caldwell.

Lord Advocate Frank Mulholland instructed the national force to re-open the inquiry after the Crown Office's latest cold case review.

It is understood detectives will be drawn from outside Glasgow and Lanarkshire in a bid to ensure those investigating the killing have an open mind on what happened to the 27-year-old.

Ms Caldwell was last seen leaving a homeless hostel in Glasgow on April 4, 2005. Her naked body was found a month later 30 miles away in woods near Roberton, Lanarkshire.

Originally from Erskine, Renfrewshire, Ms Caldwell had developed a heroin habit after becoming depressed following her sister's death some years earlier. Before her murder, she had been paying for her habit on Glasgow's main off-street prostitution drag.

A Crown Office spokesman: "The Lord Advocate has instructed Police Scotland to carry out a reinvestigation into the murder of Emma Caldwell.

"The decision followed careful consideration of the case by Crown Counsel, the most senior lawyers in Crown Office."

The re-investigation comes after a review by the Cold Case Unit. It is notoriously difficult to achieve results years after a crime. The unit scored its first major success last year when it secured the conviction of soldier John Docherty for the 1986 murder of teenager Elaine Doyle.

This year another prosecution based on a cold case review, for the killing of Martin Toner in 2004, also in Inverclyde, ended with a not-proven verdict.

Ms Caldwell's family - who have highlighted how heroin can destroy people even from loving, secure homes - have been told of the new investigation.

Crown officials always insist no unsolved murder is every closed. But their Cold Case Unit and Police Scotland, often using ever improving technology, has looked at a series of cases, of high and low profile, to see if criminal proceedings can be brought.

Forensic advances last year helped to insure the conviction of Angus Sinclair for the 1973 World's End murders in Edinburgh.

Some 70 murders remain unsolved in Scotland, including the 1957 disappearance of Moira Anderson, which is also under re-investigation.

Assistant Chief Constable Malcolm Graham said: "Police Scotland is at an advanced stage in its preparations to undertake a re-investigation into the murder of Emma Caldwell.

"All necessary resources with the appropriate skills and experience will be deployed to ensure a thorough re-investigation takes place.

"Senior officers will meet the family to outline the plans in place and provide information on areas to be explored in an attempt to identify new evidential opportunities.

"Detectives will be using the most up-to-date investigative techniques, particularly those in forensics, within which there have been significant advances in recent years."

The old Strathclyde Police reportedly spent £4m on a previous investigation in to the death of Ms Caldwell.

Four Turkish men were eventually charged but the case against them collapsed.

Ms Caldwell's father, William, died in 2011 without seeing justice for his daughter.

Her mother, Margaret, said: "It has been so hard over the years and part of me just wants to let it go but she was my little girl, is still my little girl, and we need to know the truth. I hope they will properly look at the evidence again."