The leader of the Scottish Liberal Democrats has called on the Crown office to confirm when an investigation into the deaths of two people in a crash on the M9 will be published.

Lamara Bell, 25, and boyfriend John Yuill, 28, died after they crashed off the motorway near Stirling on July 5 last year.

They lay undiscovered in the car for three days and Ms Bell was still alive when found, but died later in hospital.

An investigation was sparked after it was revealed a witness had phoned police on the day of the crash to say he had seen the couple's Renault Clio come off the road.

However, the call was not logged and emergency services did not look for the couple until days after the incident.

Father-of-three Mr Yuill is understood to have died on impact. Mother-of-two Ms Bell was found injured and badly dehydrated in the wreckage and died seven days later in hospital.

The Lord Advocate Frank Mulholland was handed an interim report from the Police Investigations and Review Commissioner (Pirc) into the deaths of the couple in December.

At the time, Pirc confirmed that Mr Mulholland would consider its findings and it would undertake further enquiries as required.

No date has been confirmed for the publication of the report.

Willie Rennie MSP has written to Mr Mulholland requesting an update on the investigation and a publication date.

In the letter, Mr Rennie says: "I would be grateful for any indication that you can provide as to when this investigation will conclude and when you will be in a position to publish its findings.

"I would also appreciate it if you could outline the range of options that will be available to you once you have received the final report from the Pirc.

"As you will appreciate, once all the facts have been established, there are compelling reasons for information to be made available as soon as possible. And I hope that you agree with me about the need for as much transparency as possible.

"I believe this final report will be an important step to attaining closure for those involved in this tragic incident.

"Police Scotland also continues to progress its plans to shut more control rooms and service centres. It is essential that the national force has every opportunity to learn from what occurred in this tragic case to ensure that it never happens again."

A spokeswoman for the Crown office said: "The Lord Advocate has received the letter from Willie Rennie and will respond in due course."