The MPs' register of interests stated the accommodation had a "value" of £15,000 and was covered by Carphone Warehouse co-founder David Ross.

Mr Ross initially said he had not paid for the holiday, but in a clarification insisted the register "is correct" and he had "facilitated accommodation".

Downing Street said the trip had been properly registered.

The prime minister took the holiday to Mustique, a private island that is part of St Vincent and the Grenadines, with girlfriend Carrie Symonds between Boxing Day 2019 and 5 January 2020.

Labour's Jon Trickett has now asked the parliamentary commissioner for standards to investigate who paid for it.

In a letter to the watchdog, the shadow Cabinet Office minister said: "The code of conduct requires members to provide the name of the person or organisation that actually funded a donation."

He said that the "evidence suggests it was not David Ross" who funded the donation and that the entry made by the prime minister in the MPs' register of interests "appears to be incorrect".

Mr Trickett said a number of questions needed "urgently answering", including "the true source of the £15,000 donation" and "did the PM knowingly make a false entry into the register".

"Transparency is crucial to ensuring that the public have confidence that elected Members of this House have not been unduly influence by any donations or gifts that they may receive," he added.

Mr Johnson's entry in the register of interests says Mr Ross donated accommodation "for a private holiday for my partner and me, value £15,000".

But a spokesman for 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."

A later statement from the spokesman added: "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."

A Downing Street spokesperson said: "All transparency requirements have been followed, as set out in the Register of Members' Financial Interests".

A spokeswoman for the standards commissioner said the office could not confirm whether an investigation had been opened into the prime minister.

She explained this was due to a decision by MPs in 2018 to allow colleagues being investigated to remain anonymous.

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.

Mr Ross has been a long-standing donor to the Conservative Party, pledging £250,000 in the last election campaign.

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.

In April 2019 Mr Johnson was rebuked by the parliamentary commissioner for standards for failing to register a share of a Somerset property within 28 days of acquiring it.