A controversial decision by the Crown Prosecution Service (CPS) not to prosecute Lord Janner of Braunstone over 22 alleged historical child sex crimes is being reviewed. A group claiming the ex-Labour MP sexually abused them has claimed credit for the reversal.

The Director of Public Prosecutions had said Lord Janner, 86, would not be prosecuted, despite sufficient evidence to bring a case, due to his being too ill with dementia.

An unnamed QC has been asked to re-examine the case, a CPS spokesman said on Friday. The move could overturn the block on court action.

The push for an independent inquiry came after Janner’s alleged victims demanded it and Leicestershire police, which had investigated claims against the peer, said they were considering a legal challenge. In a statement, the CPS spokesperson confirmed that the QC who has had “no previous involvement with the decision” is reviewing the matter.

Defending her decision following calls for a review last month, Director of Public Prosecutions Alison Saunders told the BBC that Lord Janner’s dementia was so severe that he could “play no part in a trial”.

Ms Saunders said that she had written to the alleged victims to inform them they have the right to a review.

Simon Danczuk, the Labour MP, told the Guardian a review prompts new questions which must be answered.

“I welcome this but I am worried that the CPS is commissioning a review after they have frankly got this whole episode wrong from the very beginning,” he said. “I hope it is entirely independent of previous decisions by Saunders.

“She sat on the police file for nine months before announcing a muddleheaded decision when parliament was not sitting and is now announcing this just as MPs prepare to come back. I hope we are not seeing political manipulation with a small P.”

Janner was accused of a catalogue of abuse against young boys, and more than a dozen people came forward to claim he abused them during the 1960s, 1970s and 1980s. His family have always strenuously denied the allegations and claimed he was entirely innocent of any wrongdoing.

“As the Crown Prosecution Service indicated today, this decision does not mean or imply that any of the allegations that have been made are established or that Lord Janner is guilty of any offence,” a statement released last month said.

The new review is expected to be complete by September.