Ex-child protection officer Peter McKelvie has resigned as an adviser to the Independent Inquiry into Child Sexual Abuse (IICSA), the inquiry says.
Mr McKelvie said he left after being told he may be required as a witness during the inquiry's investigations.
His information had led to Labour MP Tom Watson raising concerns over whether a minister had links to a past paedophile ring.
It was later reported that the police had found no evidence for such a claim.
The IICSA inquiry, sparked by claims of paedophiles operating in Westminster in the 1980s, will investigate whether "state and non-state institutions have failed in their duty of care to protect children from sexual abuse and exploitation" in England and Wales.
Mr McKelvie said he had "reluctantly" resigned after being "advised that I am likely to be required as a witness in the inquiry's investigations, and that the inquiry may need to examine my work in pursuing allegations of [child sexual abuse]".
Justice Lowell Goddard, who is leading IICSA, thanked Mr McKelvie for his work on the Victims and Survivors Consultative Panel.
She added: "I would also like to take this opportunity to stress that allegations concerning child sexual abuse related to Westminster are only one component of the inquiry's work.
"As I said in my opening statement the inquiry's terms of reference go far broader than this and encompass all institutions within England and Wales. This important work continues."
The BBC reported on Thursday that emails unearthed by the BBC's Panorama programme showed detectives dismissed the allegations made by Mr Watson in October 2012, two months after the Commons statement.
The files on which Mr Watson based his statement were seized as part of an investigation involving Mr McKelvie, into the paedophile ring centred on senior social worker Peter Righton in the early 1990s.
Mr Watson was briefed on their content, before making his statement, by Mr McKelvie.
Mr McKelvie has told the BBC he did not claim to Mr Watson that there had been a "Westminster paedophile ring" and pointed out that the police investigations which followed the Commons statement had led to convictions of two men who were part of the original paedophile ring.
It also emerged on Thursday that child sex abuse victims have been asked to resubmit information to the inquiry after it was accidentally deleted.
Online forms were deleted due to a "change in our website address", the Goddard Inquiry into historical abuse said on its website.