The Victims' Commissioner is to warn against any move for those who report that they have suffered sexual abuse to be routinely referred to as "complainants".
Baroness Newlove will voice her opposition on Friday to recommendations made in a review of Scotland Yard's doomed investigation into allegations of a VIP paedophile ring.
In the report published last month, retired High Court judge Sir Richard Henriques said those who make complaints should be referred to by the Met Police as "complainants" and not as "victims" throughout both the investigative and judicial process.
He also recommended that an instruction to "believe a victim's account" should cease.
Baroness Newlove will say the report "gives me serious cause for concern".
She will say: "I believe that if these recommendations are accepted, the police will be taking a significant step backwards, undoing much of the progress made to give victims the confidence to come forward."
The Commissioner will go on to say: "Belief and acceptance of the events a victim has experienced, makes up a huge part of helping a victim towards their recovery.
"An important part of this is for victims to know that they are not 'complainants'. I simply cannot understand this terminology. Victims of crime, any crime, whether they have visible scars or not, are just that - victims.
"They do not become victims only at the point when a perpetrator is convicted.
"They have not asked for the crime to be committed. Nor are they asking for a complaint to be resolved. It is the State that brings a case against a suspect, and it is the State who brings an offender to justice."
Giving a speech in Rotherham, Baroness Newlove will say a "conscious decision" by many authorities to change their approach "led to a climate where victims feel able to report the crimes they have suffered and in an environment where they feel safe".
She will add: "The police especially, have made great strides in changing the way they deal with victims and survivors of sexual abuse. One of the most important changes is that they ensure victims are listened to and that their accounts are believed."
Sir Richard's report identified a string of failings in the Met's £2.5 million Operation Midland, which ended earlier this year without a single arrest.