Boris Johnson is under investigation by Westminster’s parliamentary watchdog for his £15,000 Caribbean holiday to Mustique with his fiancee Carrie Symonds.

The Parliamentary Commissioner for Standards Kathryn Stone announced today that it was looking at Johnson’s holiday over the 2019/20 New Year period, after an initial saga over who paid for the holiday.

The probe is the second in three weeks into sleaze allegations against Johnson.

The holiday was listed as a “benefit in kind” by the Prime Minister in the MPs’ register of interests, with the donor listed as Carphone Warehouse co-founder David Ross.

However, Ross denied he had paid for the trip in an interview with the Daily Mail and said that he had only put the Prime Minister in touch with accommodation providers on the island.

Stone’s probe will concentrate on who actually paid for the holiday and whether Johnson breached Paragraph 14 of the MPs’ standards code, which states that parliamentarians must “fulfil conscientiously the requirements of the House in respect of the registration of interests.”

The investigation comes after Labour wrote to the Parliamentary Commissioner for Standards to ask for a full investigation into who actually paid for the holiday.

Number 10 has consistently denied any wrongdoing or incorrect listing of the holiday in the register of interests.

A spokersperson for Ross 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 new probe will pile more pressure on the Prime Minister after the Electoral Commission two weeks ago announced it was probing the renovation of his Downing Street flat.

The £58,000 flat renovation has now been paid by Johnson, but only after originally paid for by the Tory party.

It is believed the initial funds were given to the party through an undisclosed donation – a breach of commission rules as all donations must be reported.

The Electoral Commission investigation is into the Conservative party and not the Prime Minister directly.