Boris Johnson is facing questions over who paid for his £15,000 Caribbean holiday with his girlfriend Carrie Symonds after a Tory donor denied funding the new year trip.

The prime minister claimed the luxury trip to Mustique was paid for by David Ross, a Conservative donor who co-founded the Carphone Warehouse chain, in an entry into the Commons register of members’ interests.

But the businessman denied funding the trip, prompting calls from Labour for Mr Johnson to “fess up” or face a parliamentary inquiry.

The prime minister came under fire at the time for failing to return early from the post-election break to deal with the crisis in Iran.

Mr Johnson’s entry in the register stated: “Accommodation for a private holiday for my partner and me, value £15,000.” The private holiday lasted from 26 December to 5 January.

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Mr Ross was one of Mr Johnson’s aides in City Hall and was appointed to the Olympics organising committee. But he resigned from the roles, and his company, over a share scandal in 2008.

A spokesman for the businessman said: “Boris Johnson did not stay in David Ross’s house. 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.”

Asked about Mr Johnson’s declaration, the spokesman said: “I believe it is a mistake.” He added that Mr Ross had “not put his hand in his pocket whatsoever and can obviously prove that – [he] most definitely did not pay anything and it was not his house.

“It was a house that was rented but the people could not turn up, so Boris Johnson got the use of it.”

Labour has called for the PM to provide answers over the trip or else face a parliamentary inquiry.

“Boris Johnson must come clean about who has paid for his luxury trip,” said Jon Trickett, the party’s shadow cabinet office minister.

“If he fails to do so, the Parliamentary Commissioner for Standards should step in and make him fess up.

“The public deserves to know who is paying for their prime minister’s jaunts.”

Tory MP Tobias Ellwood dismissed questions about the trip, saying it was more important to focus on the cabinet reshuffle on Thursday.

He told BBC Radio 4’s Today programme: “We have a very transparent system to show who does actually pay.

“If that’s an important question for you then by all means ask it but for me, particularly on a day like this when I see us really being able to reshape Britain and determine where the Conservative Party will go potentially over the next 10 years years, it is not one I am willing to answer.”

A No 10 spokeswoman said: “All transparency requirements have been followed, as set out in the register of members’ financial interests.”

Mr Johnson had already faced criticism at the time for failing to cut the festive break short following the break-out of international tensions when the US killed Iranian general Qassem Soleimani on 3 January.