A watchdog is investigating whether the DUP's Ian Paisley broke the MPs' code of conduct over alleged "all expenses paid" holidays to Sri Lanka.
Mr Paisley reported himself to the Parliamentary Commissioner for Standards, Kathryn Hudson, last week.
It followed a Daily Telegraph claim he had two family holidays that were not declared in his register of interests.
Mr Paisley has denied the claims and described the report as "devoid of fact or logic" and "defamatory".
The commissioner took a number of days to assess the matter before confirming that a formal inquiry is now under way.
Mr Paisley is being investigated for an "alleged breach of paragraph 13 of the Code of Conduct".
Paragraph 13 states: "Members shall fulfil conscientiously the requirements of the House in respect of the registration of interests in the Register of Members' Financial Interests.
"They shall always be open and frank in drawing attention to any relevant interest in any proceeding of the House or its committees, and in any communications with ministers, members, public officials or public office holders."
A statement from Mr Paisley's solicitor said he had referred the matter, and a full explanation, to Ms Hudson.
The North Antrim MP is a son of the founder of the Democratic Unionist Party (DUP), the late Ian Paisley Sr.