A former director of public prosecutions has said he was “never informed” about an investigation into historical child sex allegations against former Labour MP Lord Janner.
Lord Macdonald QC said it was of “great regret” that regional Crown Prosecution Service lawyers had failed to refer the case to him in 2007.
He said he would have given the allegations his “close attention”.
Cardiff-born peer Lord Janner, 86, has always denied any wrongdoing.
The abuse allegations against the Labour peer relate to children’s homes in Leicestershire in the 1970s and 80s.
On Thursday, current Director of Public Prosecutions Alison Saunders announced that there was enough evidence to bring charges against Lord Janner but that this would not happen because he was now too sick to stand trial. He was diagnosed with Alzheimer’s disease in 2009.
Ms Saunders also said the CPS had been “wrong” not to prosecute following investigations in 1991 and 2007.
On Saturday, speaking to the BBC on a visit to North Cornwall, Home Secretary Theresa May, said she was “very concerned” over the decision not to prosecute the former Labour MP.
She said: “It’s not my decision, it’s an entirely independent decision for the director of public prosecutions.
“I’ve been very clear, in everything I’ve said so far about the child sex abuse issue… that I expect to see justice done.”
Speaking to BBC Radio 4’s Today programme on Saturday, Lord Macdonald said he had never been notified about the 2007 police investigation, for reasons which he “did not understand”.
“It was apparently a serious police investigation, and it should have been absolutely clear to the lawyers in Leicestershire that this case should have been sent to London.
“I would have undoubtedly taken a look at it personally, and would have undoubtedly myself have considered the question as to whether or not Lord Janner should be charged, and I very much regret that that didn’t happen.”
The decision on whether or not Lord Janner should now face charges was “reasonably finely balanced”, he said, adding: “I think you could justify a decision going either way frankly.”
He said he had not seen all the material relating to the case, but that “in the light of the obvious errors of the CPS in the past with this case” it might have been better to resolve it in open court.
Analysis by Clive Coleman, BBC legal correspondent
On the decision not to prosecute Greville Janner, Lord Macdonald acknowledges that he hasn’t seen the evidence, and doesn’t go so far as to say Alison Saunders got it wrong, though he says you could justify a decision going either way.
And unlike his successor, he favours a limited court hearing to establish the facts of the alleged abuse.
Lord Macdonald also says that protocols were broken when regional prosecutors failed to refer the file on Greville Janner to specialist senior CPS lawyers in London and to himself in 2007. He says that they should emphatically not have dismissed the case.
This morning will be painful for Alison Saunders.
She’s had a far from ringing endorsement from her predecessor on the Janner case, and a mauling in the press over the failures of Operation Elveden and its prosecution of journalists for paying public officials.
Lord Janner was first interviewed in 1991 when his name was mentioned in the trial of Frank Beck, one of Britain’s most notorious paedophiles who was jailed for abusing boys in his care at Leicestershire children’s homes.
The politician was accused of grooming and abusing a boy aged between 13 and 15, but the CPS did not pursue the case.
He was investigated again in 2002 and 2006-7 when fresh allegations surfaced, but no action was taken, and the case was dropped.
In the course of its most recent investigation launched in 2013, Leicestershire Police interviewed more than 2,000 people and a “comprehensive file of evidence” was submitted to the CPS.
More than a dozen individuals made allegations to police relating to Lord Janner.
The “core allegation” was that as MP for Leicester West at the time, Lord Janner befriended the manager of a children’s care home to allow him access to children so he could “perpetrate serious sexual offences on children”, the CPS said.
Lord Janner denied this allegation. He is “entirely innocent of any wrongdoing”, the peer’s family has said.
Prosecutors said that despite having enough evidence to charge the peer with 22 sex offences, he was now too unwell to stand trial.
A retired High Court judge will now review the CPS’s handling of the case.