A formal investigation is underway into Boris Johnson's 2020 holiday to the Caribbean island of Mustique, the Parliamentary Commissioner for Standards confirmed today.
The watchdog's probe will seek to establish who paid for Mr Johnson and partner Carrie Symond's trip.
However, the watchdog does not appear to be investigating the PM over the so-called 'wallpaper-gate' row relating to the refurbishment of his Downing Street flat.
Labour MPs had urged the standards commissioner, Kathryn Stone, to examine the matter but it is not included in a new list of confirmed investigations.
Mr Johnson is being investigated over the holiday under paragraph 14 of the parliamentary code of conduct which states MPs 'shall always be open and frank in drawing attention to any relevant interest'.
The Parliamentary Commissioner for Standards today confirmed a prove has been launched in Boris Johnson and Carrie Symonds' trip to the island of Mustique in 2020. The PM and his partner are pictured in London last Thursday
The PM and Ms Symonds accepted accommodation for a private winter break in St Vincent and the Grenadines as a post-election victory escape. The island of Mustique is pictured
Mr Johnson and Ms Symonds are said to have stayed at Indigo Villa on the island of Mustique
The probe is in specific relation to 'visits outside the UK' in 2020. The rules state that any interest should be registered if it is 'over £300 if not wholly borne by Member or public funds'.
Should Mr Johnson be judged to have broken the code of conduct, the watchdog has the power to impose sanctions ranging from ordering him to apologise to the House of Commons or even suspending him from Parliament for a number of days.
Responding to the probe being confirmed, the Prime Minister’s Official Spokesman said: ‘I think the Prime Minister transparently declared the benefit in kind in the Commons’ register of interests so I would point you to that declaration and I don’t have anything more to add to that.’
Confirmation of the formal investigation into the trip comes more than a year after reports that the PM was facing a probe by the watchdog into the matter.
The PM and Ms Symonds accepted accommodation for a private winter break in St Vincent and the Grenadines as a post-election victory escape.
But confusion reigned after Mr Johnson declared in the register of MPs' interests that he had accepted 'accommodation for a private holiday for my partner and me, value £15,000', citing businessman David Ross as the provider.
A spokesman for Mr Ross then denied that the co-founder of the Carphone Warehouse had stumped up any money.
But the Conservative Party donor's spokesman later clarified his stance, agreeing it was a 'benefit in kind' to the PM and Ms Symonds during their private break to the island of Mustique.
The Observer reported last year that the Parliamentary Commissioner for Standards had decided to pursue an official inquiry into Mr Johnson, and had requested information from the PM and Mr Ross.
Labour had called for an inquiry into how the PM came to enjoy the free provision of a five-figure villa.
In a letter to the commissioner sent last year, the then-shadow Cabinet Office minister Jon Trickett said: 'The Code of Conduct requires members to provide the name of the person or organisation that actually funded a donation.
'The evidence now suggests it was not David Ross. The entry made by the Prime Minister therefore appears to be incorrect.'
The trip provided Mr Johnson with a break after the 2019 general election campaign, which produced a Conservative landslide for the first time since the 1980s.
Mr Johnson and Ms Symonds were said to have stayed at Indigo Villa on the island of Mustique in the Caribbean
Labour wrote to the Parliamentary Commissioner for Standards in February last year calling for a probe into the PM's trip
But he faced criticism for failing to cut the holiday over the New Year period short when international tensions rose after the US killed Iranian general Qassem Soleimani on January 3.
Labour MP Margaret Hodge last month urged the parliamentary watchdog to investigate the refurbishment of the PM's Downing Street flat.
Mr Johnson has said he 'personally' paid for the renovations but has refused to say whether he received an initial donation from the Conservative Party to cover the costs.
The refurbishment is already subject to an Electoral Commission investigation while Cabinet Secretary Simon Case and Lord Geidt, the new Independent Adviser on Ministers’ Interests, are also conducting their own probes.