Boris Johnson is under investigation by the parliamentary standards watchdog over claims he may have broken rules in the way he declared a new-year holiday in the Caribbean.

Kathryn Stone, the parliamentary standards commissioner, is looking at the UK prime minister’s declaration of a luxury break on the island of Mustique in early 2020, which he took with his partner Carrie Symonds.

Downing Street has insisted that Johnson had complied with “all relevant transparency requirements” for the holiday — which he took after his election victory in December 2019 — in the register of MPs’ interests.

In the entry Johnson said he accepted “accommodation for a private holiday for my partner and me, value £15,000”, and said it had been provided by the Conservative party donor David Ross.

In February 2020 a spokesman for Ross, co-founder of Carphone Warehouse, told the Daily Mail: “Boris wanted some help to find somewhere in Mustique, David called the company who run all the villas and somebody had dropped out.

“So Boris got the use of a villa that was worth £15,000, but David Ross did not pay any monies whatsoever for this.”

A spokesman for Ross said on Monday: “Mr Ross facilitated accommodation for Mr Johnson on Mustique valued at £15,000. Therefore, this is a benefit in kind from Mr Ross to Mr Johnson, and Mr Johnson’s declaration to the House of Commons is correct.”

Johnson’s spokesman said the prime minister had “transparently declared a benefit in kind in the Commons register of interests”, adding that Ross’s spokesman had said the prime minister’s entry was correct.

But Labour’s deputy leader Angela Rayner said the inquiry into the holiday was more evidence of “sleaze and dodgy dealings” following the dispute over who initially paid for the refurbishment of Johnson’s Downing Street flat.

“The public have a right to know who paid for Boris Johnson’s luxury Caribbean holiday and the renovation of his flat,” she said. “Most importantly, we need to know what these donors were promised or expected in return for their generosity.”

Stone confirmed on Monday she was looking into the Mustique holiday as one of a number of investigations launched into the conduct of nine MPs.

The Commons code of conduct states that MPs must be “open and frank in drawing attention to any relevant interest”. Johnson could be ordered to apologise to the Commons if he was deemed to have broken the rules; serious offences can see an MP suspended.

Stone has also been asked by Labour MP Margaret Hodge to look into Johnson’s financial arrangements regarding the refurbishment of the Downing Street flat. The prime minister says he paid for the work, but has refused to say who initially met the bill.