DUP MP Ian Paisley has totally denied "defamatory inferences" in a report that he did not declare trips paid for by the Sri Lankan government.
The party founder's son has referred the matter with a "full explanation" to the Parliamentary Standards Commissioner.
The Daily Telegraph reported that he accepted two holidays to the Indian Ocean island in 2013 for him and his family.
A letter from Mr Paisley's solicitor, Paul Tweed, said: "My client totally denies the defamatory inferences arising from the article in today's Daily Telegraph, including those relating to his registration obligations as an MP.
"He has now referred this matter, and a full explanation, to the Parliamentary Commissioner for Standards."
Mr Paisley is one of 10 pro-Brexit DUP MPs helping to prop up Theresa May's Tory administration after her snap election left her with no overall majority.
He tweeted: "The Daily Telegraph article is defamatory. It is devoid of fact or logic. Referred to my lawyer.
"I will refer myself to the Parliamentary Commissioner for Standards."
He posted a picture this week on the social networking site of himself meeting Sri Lankan High Commissioner Amari Wijewardene "to discuss NI-Sri Lanka trade deal after Brexit".
Two days later he tweeted a picture of himself with International Trade Secretary Liam Fox "discussing our trade agreements post Brexit".
The House of Commons Code of Conduct states that MPs must declare any visit to a destination outside the UK which "relates in any way to their membership of the House or to their parliamentary or political activities" and which cost more than £300, unless they have paid for it themselves or out of parliamentary or party funds.
The rules state that MPs do not have to register family holidays, as long as they are "wholly unconnected with membership of the House or with the member's parliamentary or political activities".
Entries in the Register of Members' Interests should cover the cost of travel, hotels, meals, hospitality and car hire, and repeat visits should be registered if their combined value comes to more than £300.
Mr Paisley's register entries include a trade mission to Sri Lanka in 2012, as well as a second trip to the island that year as part of a cross-party parliamentary delegation examining post-war reconstruction, funded to the tune of £3,200 by the Colombo government.
There is no mention of the alleged trips in 2013.
Complaints over alleged breaches of the Code are investigated by Parliamentary Standards Commissioner Kathryn Hudson, who reports to the Commons Standards Committee.
This committee, made up of MPs and lay members, has the power to recommend sanctions such as requiring an apology or temporary suspension, subject to a vote in the Commons.