Inquiry: Police have launched a new investigation into allegations that Labour peer Lord Janner (pictured) sexually abused a teenage boy in Scotland in the 1970s

Detectives have launched a dramatic new investigation into allegations that Labour peer Lord Janner sexually abused a teenage boy.

Officers will also look at whether he engineered a cover-up when police first received the complaint 24 years ago.

The surprise development will pile more pressure on the Crown Prosecution Service in England over its controversial decision not to bring charges against the former Labour MP because he has dementia.

The new police investigation is being carried out in Scotland, which has its own legal system, raising the possibility that the peer could still be prosecuted north of the border.

Janner, 86, stands accused of abusing vulnerable youngsters at care homes in his former Leicester constituency between 1969 and 1988.

The CPS announced in April that there was enough evidence to charge him with 22 historic child sex offences – but ruled he should not face trial because he is suffering from advanced Alzheimer’s disease.

But now detectives have launched an inquiry into claims that Janner took a boy with him to Scotland in the 1970s and subjected him to serious sexual assaults there. The alleged victim filed a complaint at an Edinburgh police station in 1991, but the Crown Office – Scotland’s prosecution service – maintains that its officials were never told about the allegations.

Police Scotland spent several weeks combing through archived files and have now found documents relating to the case, sparking the renewed investigation.

A police source said: ‘We have located the relevant files which show a complaint was made and now a live investigation is under way. We now have to look at this case applying 2015 policing principles to it – to ask whether, if the same allegations were made today, how would they be treated?

‘And was this claim investigated in the correct way, or in a way that would be acceptable today, back in 1991? If not, why not?’

It is understood police chiefs were surprised that the Crown Office moved so quickly to deny publicly that its officials had any record of the allegation from the time.

Detective Chief Superintendent Lesley Boal, of Police Scotland, told the Mail: ‘We are conducting an investigation into an historic complaint and, as such it, would be inappropriate to comment.

‘Police Scotland is absolutely committed to preventing all forms of child abuse and to keeping children safe while bringing perpetrators of abuse to justice, regardless of the passage of time.’ Last night the Crown Office said it had ‘not received a report in relation to this matter’.

The surprise development will pile more pressure on the CPS in England over its controversial decision not to bring charges against the former Labour MP (pictured with Tony Blair in 1997) because he has dementia

Detectives in Scotland are now liaising with lawyers representing the man, who claims he was abused by Janner during the trip to Scotland, and keeping them informed of developments.

The first allegations against Janner emerged during the trial of Frank Beck, a social worker in Leicestershire who was found guilty of child abuse in 1991.

Leicestershire Police began an investigation, but prosecutors said no further action would be taken. In July 1991, the alleged victim who gave evidence during the Beck trial – then in his 30s – made a report in person at an Edinburgh police station. The latest revelations come amid mounting evidence Janner’s alleged paedophile activities may have gone unchecked because of his powerful police contacts.

Kelvyn Ashby, a former detective inspector in Leicester, has claimed that senior officers forced him to shelve his inquiry into the peer, who was a Leicester MP for nearly 30 years. He said the alleged abuse was said to have taken place during a trip around Scotland.

Was this claim investigated in the correct way, or in a way that would be acceptable today, back in 1991? If not, why not? Police source

He added: ‘My only recollection is that we verified that the tour took place. I think it was some sort of official tour for the Labour Party. It must have been quite a rarity for an English Parliamentarian to be going on a tour of Scotland.’

He insisted his officers would have contacted the relevant Scottish force to keep it fully updated about any alleged crimes committed within its jurisdiction.

Liz Dux, who is representing the man who made the allegations relating to Scotland, said: ‘I’d be keen to see every allegation that has not been properly investigated looked at again.’

The solicitor, from law firm Slater and Gordon, said lawyers would pursue civil claims against Lord Janner on behalf of his alleged victims, even if an on-going independent review upholds the CPS’s decision not to prosecute. The review is likely to be completed by the end of this month.

Lord Janner’s family have insisted he is ‘entirely innocent of any wrong-doing’. Three further investigations into the peer were launched in 2002, 2006 and 2013 – but the CPS has admitted that he should have been charged on each occasion.