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

A spokesman for Mr Ross said the register "is correct" and he "facilitated accommodation" for the PM.

Downing Street said the trip had been properly registered.

The register also shows earnings Mr Johnson received last year before becoming PM, including more than £327,000 for seven speaking engagements, one of which was a three-hour speech where he was paid £122,899.74.

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 holiday accommodation, 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 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."

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 Johnson also declared payments he had received prior to becoming prime minister, including book royalties and hundreds of thousands of pounds for speaking engagements.

In the first six months of 2019, Mr Johnson earned more than £327,000 for the seven speaking engagements, which lasted a total of 17.5 hours.

He was also paid £22,916.66 a month for his column in the Daily Telegraph newspaper, which was published weekly.