Boris Johnson faces a probe by Parliament's sleaze watchdog over his £15,000 winter break to the Caribbean, according to reports.
The Prime Minister and his partner Carrie Symonds stayed on the private island of Mustique after the election.
But confusion reigned after Mr Johnson declared in the register of MPs' interests that he had accepted "accommodation for a private holiday for my partner and me, value £15,000", citing businessman and long-term Tory donor David Ross as the provider.
A spokesman for Mr Ross then denied that the co-founder of the Carphone Warehouse had stumped up any money.
But the Conservative Party donor's spokesman later clarified his stance, agreeing it was a "benefit in kind" to the PM and Ms Symonds during their private break to the island of Mustique.
The Observer said the Parliamentary Commissioner for Standards, Kathryn Stone, decided to pursue an official inquiry into Mr Johnson on Wednesday, and has requested information from the PM and Mr Ross.
It follows calls from Labour for an inquiry into how the PM came to enjoy the free provision of a five-figure villa.
In a letter to the commissioner last month, shadow Cabinet Office minister Jon Trickett said: "The Code of Conduct requires members to provide the name of the person or organisation that actually funded a donation.
"The evidence now suggests it was not David Ross. The entry made by the Prime Minister therefore appears to be incorrect."
Sarah Richardson, who owns the hideaway on the paradise island of Mustique where the PM spent 10 days, confirmed she and her husband Craig “got paid” for the stay last month.
But she said she had “no idea” who covered the cost, as the arrangement was handled by the private island’s management company.
The trip provided Mr Johnson with a break after the election campaign, but he faced criticism for failing to cut the festive holiday short when international tensions rose after the US killed Iranian general Qassem Soleimani on January 3.
The Commissioner would investigate whether Mr Johnson has broken the MPs' code of conduct, and forward any findings onto the Committee on Standards which would review the evidence and, if appropriate, recommend a penalty.
Last week Ms Symonds used Instagram to announce that the couple got engaged at the end of 2019, and are expecting a baby early this summer.
The boy or girl will be the sixth child with three different women for the Prime Minister.
He will become the first British prime minister to marry in office for 250 years and the first to have a child in office since David and Samantha Cameron welcomed their daughter Florence in 2010.
Downing Street declined to comment.
