Failures by police and prosecutors meant three chances were missed to charge Lord Greville Janner over sex abuse claims, an independent report has found.

It comes as Police Scotland confirmed they are preparing to submit a report to the Crown Office in relation to an allegation that the late Labour peer, while an MP in the 1970s, brought a teenage boy to Scotland and sexually abused him. 

The alleged victim is understood to have reported the incident to police in Edinburgh in 1991 but no charges were brought. 

However, Police Scotland reportedly combed the archives last year and uncovered documents relating to the case, triggering a live investigation.

A spokesman for Police Scotland said: "As a result of information received Police Scotland made enquiries in relation to reported non recent abuse involving the late Lord Janner.

"Police Scotland confirmed the investigation has recently concluded.

"Police Scotland encourages victims of sexual abuse to come forward and report instances of abuse regardless of when and where it occurred."

Detectives in the case are said to be looking into whether Lord Janner used his political influence and police contacts to stifle an investigation into his alleged crimes. 

Meanwhile, it has emerged that a number of opporunities were missed to prosecute the peer.

Alleged victims were left ''devastated'' after a criminal case was dropped on Friday, decades after they first accused him of abuse.

Today [Tuesday], a report by High Court judge Sir Richard Henriques found that:

:: In 1991, a decision not to charge the peer was wrong and there was enough evidence to charge him with indecent assault and buggery. The charging decision was taken after an "inadequate" police investigation and should have been delayed.

:: In 2002, police failed to pass on further claims against the late politician to the Crown Prosecution Service

:: In 2007, again there was sufficient evidence to prosecute Lord Janner for indecent assault and buggery. He should have been arrested and interviewed and his home searched.

Director of Public Prosecutions Alison Saunders said: "The inquiry's findings that mistakes were made confirms my view that failings in the past by prosecutors and police meant that proceedings were not brought.

"It is a matter of sincere regret that on three occasions, opportunities to put the allegations against Lord Janner before a jury were not taken.

"It is important that we understand the steps which led to these decisions not to prosecute, and ensure that no such mistakes can be made again."

The peer died aged 87 in December, days after he was found unfit to stand trial after being charged with a string of sexual offences dating back to the 1960s against nine alleged victims, who were mostly under 16 at the time.

Ms Saunders was at the centre of a storm of controversy last year after originally deciding Lord Janner, who had dementia, should not be charged because of his ill health.

That decision was overturned by an independent review last year. A special hearing known as a trial of the facts had been scheduled for this year, but last week the criminal case was formally dropped following Lord Janner's death.