DUP MP Ian Paisley was given a complimentary holiday at a luxury Maldives resort months after advocating on behalf of its government.
BBC Spotlight obtained new evidence which suggested the visit was requested by the Maldivian government and facilitated by the resort owner, who had political links.
The programme examined whether the MP should have declared the trip in 2016.
He said he paid for part of the holiday and the rest was paid for by a friend.
The North Antrim MP did not reveal the identity of this friend. He said the friend was unconnected with his work and has received no benefit as a result of his work.
A DUP spokesman said: "The party officers will want to consider these very serious matters being mindful of the high standards we require of elected representatives."
Mr Paisley was recently suspended from the House of Commons for 30 days for "serious misconduct" for failing to declare two family holidays to Sri Lanka in 2013.
Ian Paisley, his wife and his two sons received full-board five-night stay at the luxury resort in October and November 2016, eight months after he was part of a controversial parliamentary visit to the islands.
Gavin Millar QC, an expert on parliamentary rules, said the Nolan principles on standards in public life placed an onus on Mr Paisley to be transparent about why he had not registered the trip to the Maldives.
"MPs should give reasons for their decisions and restrict information only when the wider public interest clearly demands," said Mr Millar.
"Now his decision in this case, his decision was not to register the benefit, after the trip in late 2016, and he has an obligation to give reasons for that decision.
"Unless he can come up with some wider public interest argument for not saying more, he should be saying significantly more about any considerations that are relevant to the motive of that source in paying that money."
Mr Paisley has been contacted by Spotlight about Mr Millar's comments but has not responded.
In a statement to Spotlight on Tuesday, Mr Paisley said: "I have responded in clear and categoric terms to your questions.
"For the record, the government of the Maldives did not organise or pay for my family vacation in 2016, which I do not intend to go into with you. I'm satisfied the vacation did not have to be recorded on the register."
Ian Paisley visited the Maldives in February 2016 with two other MPs from an All-Party Parliamentary Group.
At the time, the Maldives government, headed by President Abdulla Yameen, was being criticised by organisations including the United Nations and the Commonwealth for human rights abuses.
Mr Paisley, however, appeared to advocate on behalf of the regime, speaking out against economic sanctions.
With the other two MPs, he also visited the prison where opposition leader Mohamed Nasheed had been held, and described the conditions as quite luxurious.
Later that year Mr Paisley travelled to the Maldives again for a holiday with his wife and two children.
Spotlight's evidence, including an image which appears to be from the resort's internal records provided to the programme by an anonymous source, suggests that full board and transfers were provided complimentarily at the request of Mr Yameen's government and facilitated by the resort owner, Hussain Hilmy.
Mr Hilmy is a former minister in the Maldives government and has held a number of other important public posts.
Gavin Millar QC said that if, as Spotlight's documentary evidence suggests, the benefit was requested by the government and facilitated by Mr Hilmy, Mr Paisley should not have accepted it.
"But having accepted it, he certainly should have registered it undoubtedly.
"There are very strict rules about lobbying and creating an interest for yourself that may be perceived as lobbying. The moment you know these facts, that are disclosed in this document, the perception is that this is a reward for him having advocated for the Maldives government."
Ian Paisley has denied that the trip was connected with the government of the Maldives.
Register of members' interests
Last week, Ian Paisley told Spotlight that he had discussed the holiday in the Maldives with the Parliamentary Commissioner for Standards Kathryn Stone during her investigation into his Sri Lanka holidays.
Mr Paisley claimed that as a consequence of that conversation, he had satisfied himself he did not need to register the holiday.
After Spotlight contacted the commissioner's office, Mr Paisley got in touch with Spotlight again to clarify that he had not spoken to the commissioner as he had claimed.
He said he had in fact spoken to the parliamentary registrar who administers the register of members' interests.
Mr Millar QC said the registrar's role was limited.
"The one thing they can give you as an MP is a clear account of what the rules require and what they don't require," eh said.
"But I understand that is as far as they will go. They will not give a licence to an MP not to declare in a particular situation nor will they say you must declare in a particular situation.
The Maldives is a republic that lies south-west of the Indian sub-continent. It is made up of a chain of nearly 1,200 islands, most of them uninhabited.
Its political history has been unsettled since the electoral defeat of long-serving President Maumoon Abdul Gayoom in 2008.
"That's not how the code works. The way the code works is that it is ultimately always a matter for the MP."
Ian Paisley also told Spotlight he had evidence which, he said, "categorically disproves that the trip was connected to the government".
Two emails, which he had arranged from contacts linked to the regime and the resort.
The first was from Ahmed Shiaan, who was the Maldivian Ambassador to the UK at the time of the visit.
He said the holiday had not been arranged by the Embassy or paid for by the government of the Maldives.
'Left open to criticism'
Ian Paisley also sent Spotlight an email from the resort's commercial operating officer, Andrew Ashmore, who said invoices for the stay had been settled and paid for privately although he could not say by whom.
Political rivals have been quick to criticise Mr Paisley.
It really is time for Ian Paisley to resign. If he doesn't, then the onus is on Arlene Foster to remove him from the DUP.— Colum Eastwood (@columeastwood) December 11, 2018
Sinn Féin has written to the Parliamentary Commissioner for Standards asking her to investigate the allegations.
North Antrim MLA Philip McGuigan said there are "serious questions to answer" about Mr Paisley's relation with the government of the Maldives.
Alliance leader Naomi Long said Mr Paisley had left himself open to criticism by not revealing his friend's identity.
"If this wasn't related to parliamentary duties the easiest way to clarify that is by saying who gave the gift and why," she told BBC's Good Morning Ulster programme.
SDLP leader Colum Eastwood went further, calling for the North Antrim MP to resign.
"It really is time for Ian Paisley to resign," tweeted Mr Eastwood. "If he doesn't, then the onus is on Arlene Foster to remove him from the DUP."
Mr Eastwood later tweeted: "I've written to the Parliamentary Commissioner for Standards asking her to investigate Spotlight's allegations on Ian Paisley."
TUV leader Jim Allister said Mr Paisley now should "refer himself to the Parliamentary Standards Commissioner, so that a considered verdict can be delivered".
The UUP leader, Robin Swann, called on the commissioner to launch an investigation "urgently".
When the Daily Telegraph published revelations about his holidays to Sri Lanka in 2017, Mr Paisley initially said that the articles were "devoid of logic" and threatened legal action.
He also referred himself to the Parliamentary Commissioner for Standards.
The Commissioner, Kathryn Stone, found that Mr Paisley had failed to properly declare two holidays and engaged in paid advocacy for the Sri Lankan government.
Her findings were supported by parliament which suspended Mr Paisley from the House of Commons for 30 days.
However, a petition to trigger a by-election in his North Antrim constituency fell short by 444 votes, an outcome described by Mr Paisley as a "miracle".
On his return to the House of Commons following his suspension, he said: "A smaller man than me would have crumbled."
Mr Paisley has been MP for North Antrim since 2010, succeeding his father Ian Paisley, a former first minister of Northern Ireland and Democratic Unionist leader, who had held the seat since 1970.