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Parliament’s sleaze watchdog has dramatically intervened over plans to give anonymity to MPs being investigated for allegedly being “sex pests”.

The Commons Committee on Standards warned such a move would break with current practice at Westminster which is based on accepted principles for public life including openness, integrity, honesty and accountability.

Labour MP Sir Kevin Barron, who chairs the committee, has written to Commons Leader Andrea Leadsom to question MPs’ names being kept secret while investigations are carried out into sexual misconduct allegations.

His letter says: “The proposed arrangements for anonymity during investigations would represent a major change from the existing system (approved by the House) whereby the Commissioner announces publicly that she is conducting an investigation into a named member or members. The committee would want to consider the implications carefully.”

He also warned of the risks that such anonymity could gradually be extended to cover other allegations against MPs. “If it is proposed that the new arrangements apply to the existing code of conduct, there might be a significant backlash on the part of the media and the general public.”

The committee backed the commitment by the Commons authorities to zero tolerance of sexual harassment, harassment and bullying. But it made clear it has concerns over the planned independent complaints and grievance policy being drawn up. “The committee felt that there are a number of challenges concerning detail and process, as well as some issues of principle, that will need to be addressed,” it added.

It also said it was a priority to ensure that the new system generated confidence complaints would be dealt with “fairly and impartially”. Sir Kevin asked for an “early opportunity” for the committee to discuss the plans with parliamentary authorities.

Ms Leadsom has warned that anonymity is needed to ensure that alleged victims of sexual harrassment feel confident that they can complain without being identified, especially given that MPs have a small number of staff.

She has also stressed that the working group is seeking to follow best practice for grievance policy outside of Parliament.

A steering group has yet to decide at which stage of the process anonymity of the alleged perpetrators would end.

MPs agreed on Wednesday to press ahead with bringing in the new grievance procedure, while recognising that some amendments may be needed.