Boris Johnson under investigation over who paid for his luxury Caribbean holiday with fiancée Carrie Symonds

• The UK's sleaze watchdog is investigating the funding of a free Caribbean holiday taken by Boris Johnson last year.
• The Parliamentary Commissioner for Standards is probing who paid for Johnson's £15,000 trip to Mustique.
• Johnson declared the trip had been paid for by a Conservative donor, who initially denied paying for it, before changing his account.

The UK's sleaze watchdog is investigating whether Boris Johnson broke parliamentary rules amid conflicting accounts over who paid for a £15,000 holiday he took with his partner to the Caribbean island of Mustique last year.

The Parliamentary Commissioner for Standards on Monday announced that it was formally investigating who had paid for the trip, where Johnson was accompanied by his now-fiancee Carrie Symonds.

Kathryn Stone, the independent Parliamentary Commissioner who will carry out the investigation, will probe whether Johnson breached paragraph 14 of the parliamentary code of conduct, which states that members of parliament "shall always be open and frank in drawing attention to any relevant interest."

It is the first time a serving prime minister has been investigated by the Commissioner, the Observer reported last year.

Rules state that MPs are obliged to register any financial interests over the value of £300.

Confusion arose over who paid for the holiday after Johnson and Symonds visited Mustique between December 26 and January 5 in the wake of his crushing election victory.

The prime minister declared the holiday in a register of MPs' interests and said Conservative party donor David Ross had provided "accommodation for a private holiday for my partner and me, value £15,000."

But a spokesman for Ross, who founded the retail chain Carphone Warehouse, initially said he had not paid for the trip, and said he had only "facilitated accommodation" for the trip.

Ross subsequently retracted his claim following controversy over his comments, and said that the Mustique accommodation was a "benefit in kind" to Johnson.

A spokesman for Ross on Monday told Sky News that his position remained the same.

"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," he said.

The Observer newspaper first reported in March last year that Johnson would be investigated for the trip after the Labour Party called for a formal inquiry.

Stone, the commissioner, reportedly requested information from both Johnson and Ross, who owns a property on Mustique.

A Downing Street spokesman said: "The prime minister transparently declared the visit in the House of Commons register of interest."

"Clearly the rules are set out and it's important that everybody abides by them, as the prime minister has done throughout."