Boris Johnson claimed that his New Year trip to the Caribbean with his partner Carrie Symonds was paid for by a millionaire business friend.

The prime minister’s register of interests says his £15,0000 week-long stay in a villa on the island of Mustique was a gift from the prominent businessman and former adviser to Johnson, David Ross.

However, Ross, who founded Carphone Warehouse, said he did not pay for it and Downing Street appears to be unable to explain who actually did.

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Boris Johnson is under pressure to reveal who paid for his New Year trip to the Caribbean after the businessman who the prime minister said funded the holiday denied ever doing so.

The prime minister declared in his register of interests that his trip with his partner Carrie Symonds to a villa on the private island of Mustique was paid for by David Ross, who founded Carphone Warehouse.

Ross was previously forced to resign as Johnson’s adviser when he was mayor of London, due to a loans scandal.

The prime minister’s register of interests suggests that the accommodation came to £15,000 and was a “benefit in kind” from businessman Ross, who is also a donor to Johnson’s Conservative party.

However, Ross on Wednesday night said he neither owned or villa or paid for Johnson’s stay, the Mail reports.

A spokesperson for Ross said: “Boris Johnson did not stay in David Ross’s house.

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“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.”

Ross on Thursday subsequently released a new statement insisting that he had “facilitated” the trip, rather than paid for it.

Members of Parliament are bound by strict rules when it comes to gifts and must declare them within 28 days. Breaking the rules can result in suspension in the most serious cases.

A spokesperson for Johnson quoted by the Mail said the declaration was a “mistake” and that businessman Ross did “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.”

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

“Following media reports I would like to provide further explanation of the benefit in kind Mr Ross provided to Mr Johnson,” his spokesperson said.

“Mr Ross facilitated accommodation or 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.”

The opposition Labour Party has called for an official investigation into who paid for Johnson’s trip, and whether the prime minister falsely declared it.

“Boris Johnson must come clean about who has paid for his luxury trip,” Jon Trickett, Labour’s Shadow Cabinet Office Minister, said.

“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.”

Johnson’s trip to the Carribean was controversial at the time as he was accused of failing to return to the UK quickly enough to respond to an escalation in tensions between British ally the US and Iran.

Lord Peter Ricketts, who was the UK’s National Security Adviser between 2010 and 2012, said the prime minister should have realised that “the world doesn’t stop over Christmas and New Year,” and said Johnson was “caught short” by the diplomatic crisis.