Labour has called on Boris Johnson to clarify who paid for his Caribbean holiday over the New Year.
According to the MPs’ register of interests, the accommodation – which cost £15,000 – was covered by David Ross, the co-founder of Carphone Warehouse.
But a spokesman for Mr Ross has told the Daily Mail he did not pay for the stay and only helped arrange it.
Downing Street said the trip had been properly registered.
The prime minister took the holiday to St Vincent and the Grenadines with girlfriend Carrie Symonds between Boxing Day 2019 and 5 January 2020.
Labour’s shadow minister for the cabinet office, Jon Trickett, said Mr Johnson “must come clean” about the money, adding that if he does not, Parliament’s standards watchdog “should step in”.
“The public deserves to know who is paying for their prime minister’s jaunts,” Mr Trickett added.
The entry in Mr Johnson’s register of interests says Mr Ross donated accommodation “for a private holiday for my partner and me, value £15,000”.
But a spokesman of Mr Ross 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.”
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.
It emerged Mr Ross had used millions of pounds worth of Carphone Warehouse shares as collateral against personal loans without informing the company’s other directors – a potential breach of City rules at the time.
A Downing Street spokesperson said: “All transparency requirements have been followed, as set out in the Register of Members’ Financial Interests”.
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Mr Johnson faced criticism over his holiday for not returning sooner, after the US killing of Iranian general Qasem Soleimani raised tensions in the Middle East.
It is the first trip abroad that Mr Johnson has declared since going to Saudi Arabia in September 2018.
Only one other MP has declared a free holiday in the last year.
Mr Ross has been a long-standing donor to the Conservative Party, pledging £250,000 in the last election campaign.