An alliance of senior politicians has launched a scathing attack on the Director of Public Prosecution's decision not to prosecute the Lord Janner of Braunstone over alleged child abuse offences.

In a letter to the The Times the cross-party group demand that Alison Sauders' decision be reversed amid public concerns of a "whitewash".

The MPs, led by Simon Danczuk, of Labour, who brought to light the Cyril Smith abuse scandal in Rochdale and the subsequent cover-up, said the decision was "damaging public confidence" in the criminal justice system with her ruling.

"Have we learnt anything from the mistakes of the past?" the letter from a cross-party group of outgoing MPs asks. "As long as justice is not seen to be done and the greater public interest is not served, the public will see attempts to investigate establishment figures involved in historic child abuse as a whitewash."

Mrs Saunders announced last week that Lord Janner, QC, a Labour peer who has dementia, would not face trial over 22 serious offences against nine alleged victims in Leicestershire, dating from the 1960s to the 1980s.

Police are considering a legal challenge, and Lord Macdonald of River Glaven, a former director of public prosecutions, has also questioned the decision. A former president of the Law Society joined the chorus of criticism last night, telling The Times that the Crown Prosecution Service had failed in its duty to put suspected wrongdoers before the courts.

Alison Saunders CPS
Director of Public Prosecutions Alison Saunders refused to charge Janner with sex abuse offences Source: YouTube

Linda Lee, who now represents victims and witnesses, said: "Alison Saunders may well be right that Lord Janner is not fit to stand trial but it should not be a matter for her to decide. The matter should be brought before the courts so that the evidence may be tested in accordance with the law.

"This case clearly demonstrates why victims of abuse feel let down by the system. For years they were denied access to the courts by unaccountable public servants. Even now when it is accepted that there is sufficient evidence to charge they are once again refused that first step of a hearing to decide whether or not Lord Janner is competent to stand trial."

In a separate development, Leicestershire police told The Times that they were concerned about alleged improper attempts by a member of the Bar to influence key legal decisions in their investigation into Lord Janner, 86, and other suspects. A spokesman for the force said: "We are aware of this barrister and had concerns but were assured the CPS were dealing with this matter."

But a CPS spokesman said: "The DPP was not unduly influenced by anyone when making this decision. As head of the CPS — an independent prosecuting authority — the DPP is used to making difficult decisions and will continue to do so independently."

The peer was diagnosed with Alzheimer's disease in 2009, but continued to attend the House of Lords until December 2013. He stopped attending in the week the police searched his home and later took formal leave of absence.

That was renewed this month when Lord Janner signed a letter to House of Lords clerks.