British Prime Minister Boris Johnson came under pressure Thursday to reveal who paid for his luxury holiday in the Caribbean, with the opposition threatening a parliamentary investigation.

Johnson and his girlfriend Carrie Symonds went on a ?15,000 ($19,500, 17,900-euro) trip to the privately owned island of Mustique in Saint Vincent and the Grenadines over the New Year.

Johnson claimed it was paid for by David Ross, a donor to the PM's Conservative Party who co-founded the Carphone Warehouse mobile phone retail chain.

MPs have to register their financial interests with the parliamentary commissioner for standards.

In the register, Johnson wrote that he had accepted "accommodation for a private holiday for my partner and me, value ?15,000", as a benefit in kind.

The holiday lasted from December 26 to January 5, according to the entry, and Ross was named as the financial provider.

A spokeswoman for the PM's Downing Street office said: "All transparency requirements have been followed, as set out in the register of members' financial interests."

But a spokesman for Ross told the Daily Mail newspaper that while he helped put Johnson in touch with companies providing accommodation he did not front up money or the villa.

Johnson and is partner Carrie Symonds saw the New Year in at the privately owned island of Mustique
Johnson and is partner Carrie Symonds saw the New Year in at the privately owned island of Mustique POOL / Jeremy Selwyn

"Boris Johnson did not stay in David Ross's house," the spokesman was quoted as saying.

"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."

The spokesman later said: "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."

The main opposition Labour Party called for Johnson to provide clarity or 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."

Mustique has been a favourite getaway for the rich and famous for decades, notably the British royal family.