Prime Minister Theresa May has made assurances documents relating to the late Cyril Smith will not be withheld.
An inquiry is investigating alleged sexual abuse of boys by the former Rochdale MP in care institutions.
Wigan MP Lisa Nandy told the Commons she received a letter from Ms May stating security services work would "not prevent information being shared".
The Labour MP said Home Secretary Amber Rudd had told her some papers would be held back for "national security".
Ms Nandy said: "Last month in this house, the home secretary told me that some papers would be withheld from the Cyril Smith inquiry for national security reasons.
"This week the prime minister has written to me to say we are clear that the work of the security services will not prevent information being shared with other such inquires.
She added: "So can she can confirm to the survivors of Cyril Smith who have waited for justice for decades that she was wrong and that the prime minister is right?"
Ms Rudd replied: "I am happy to confirm the prime minister is always right and I will certainly look carefully at the letter that she has received to ensure that we comply with it."
Smith was a Liberal MP for Rochdale between 1972 and 1992.
The Independent Inquiry into Child Sexual Abuse has heard allegations of abuse at Cambridge House hostel and Knowl View residential school, where he was a governor.
It will publish its findings next year.