THE Health and Safety Executive is primed to become the third external watchdog to examine aspects of Police Scotland's handling of the M9 crash incident.

It is understood HSE officers are on “stand-by” to be called in depending on the course of the ongoing investigations.

The Police Investigations and Review Commissioner and the HM Inspectorate of Constabulary in Scotland (HMICS) are carrying out separate probes after a Falkirk couple were left in their crashed car for three days despite the alert having been raised with police.

John Yuill, 28, died in the crash on the M9 near Stirling and Lamara Bell, 25, died four days after being found following a second call to police last month.

A source close to the investigation said the potential involvement of HSE was “the elephant in the room” while it was privately confirmed HSE are poised to respond. The move was welcomed by senior politicians.

One aspect under scrutiny from the HMICS is the process of call handling and recording, said to be under pressure since the formation of a single force.

Concerns have been raised over the operation of police call centres following the creation of the national force and a programme of cuts with 1,500 jobs lost.

Scottish Labour justice spokesman Hugh Henry has raised concerns claiming call centres "are now struggling to cope at a time when 53,000 days have been lost to stress inside the force".

Five control rooms are being closed - Stirling is already shut - as part of the re-organisation and cost-cutting taking place since the single force came into being in April 2013.

Mr Henry said: "I would welcome the involvement of the HSE in the investigation of this tragic accident.

"The families of Lamara Bell and John Yuill deserve a full explanation.

"No stone should be left unturned in trying to establish the truth and the HSE has a proven expertise which could be invaluable."

Cases brought under HSE law south of the border have been raised when members of the public and police staff have been alleged to have been put at risk.

In 2007 the Metropolitan Police force was found guilty of endangering the public over the fatal shooting of a man officers mistook for a suicide bomber.

The force broke health and safety laws when officers pursued Brazilian Jean Charles de Menezes to a Tube station and shot him seven times, a jury found.

It was fined £175,000 with £385,000 costs over the 2005 shooting.

In 2003 Sir John Stevens, then Metropolitan Police Commissioner, and his predecessor Lord Condon were cleared in a £3 million case involving HSE who had claimed they failed in their duty to protect officers under their command from harm after two officers fell through roofs while chasing suspects.

The Scottish Government has said it does not believe a "systemic failure" was to blame for the initial phone call to one of its 999 centres not being passed on to officers on the ground.

However a key focus of the probe is call handling and more than 2,000 police workers and members of the public have given evidence to the inspectorate on the process.

The inspectorate is visiting all Police Scotland call centres to speak to officers and civilian staff over the next two weeks.

HSE said it was not currently active in the investigation.

A Crown Office spokesman said: “It wouldn’t be appropriate for the Crown to comment while the investigation is ongoing.”