David Cameron said it is vital police investigating historical child sex abuse allegations can go "wherever the evidence leads" to "pursue the guilty".
Mr Cameron said he was "absolutely determined" to "leave no stone unturned" to find out the truth.
"Three things need to happen: robust inquiries that get to the truth, police investigations that pursue the guilty and find out what has happened, and proper lessons learnt so we make sure these things cannot happen again. That's what will happen under my government," the Prime Minister said.
Home Secretary Theresa May is expected to announce a review into claims of historic child abuse in the Commons later today.
The Baroness says she did not consider the "difficulties" caused by her background and family connections to the child sex abuse inquiry.
Questions have been raised of former High Court judge Baroness Butler-Sloss since her appointment last week.
22 police officers are now assigned to Operation Fernbridge - the probe into claims of a Westminster cover-up of sex child abuse.