East Londonderry MP Gregory Campbell failed to declare his "remunerated positions" as an MLA and councillor to the House of Commons for almost two years, a report has found.
The DUP man apologised following a complaint which was lodged against him in April, sparking off an official two-month investigation.
The findings of a watchdog report by Parliamentary Commissioner Sir Philip Mawer into the local MP's conduct have now been published by the House of Commons Committee on Standards and Privileges.
The complainant, Derry man Noel Adams, alleged that Mr Campbell had not recorded his membership of the Northern Ireland Assembly, or of Derry City Council in the Register of Members' Interests from 2001 until March, 2007.
He further claimed that Mr Campbell failed to declare his relevant interest as a councillor at meetings of the Northern Ireland Affairs Committee held on November 29 and December 7, 2005, at which the Review of Public Administration had been considered.
Under the Guide To the Rules, members of parliament are required to submit a form detailing their registrable interests within three months of being elected or re-elected.
During the investigation, it emerged that Mr Campbell did register his role as a Derry councillor, on being elected to Parliament in 2001, while his membership of the Assembly was registered a year later.
The report states however: "Following his re-election to the House in 2005, he submitted a 'nil return' prior to publication of the first Register of the current parliament.
"That 'nil return' continued in the Register up to and including the edition published on the internet on February 6, 2007."
This was rectified by Mr Campbell on March 10 this year.
Mr Campbell said he didn't realise a declaration was required of him at a Northern Ireland Affairs Committee meeting held in Derry.
"My failure to declare membership was an oversight which I regret and apologise for."