Boris Johnson is under investigation over who paid for his Caribbean holiday with his fiancee, Carrie Symonds, during Christmas 2019. The parliamentary standards commissioner, Kathryn Stone, confirmed on Monday morning that she was investigating a possible breach of the MPs’ code of conduct. It is the latest allegation of impropriety faced by the prime minister, as the Electoral Commission continues its inquiry into the Conservative party over claims that a loan to cover works to Johnson and Symonds’ Downing Street flat was not properly declared. The “sleaze” attacks deployed against the prime minister in the run-up to last week’s local elections seem not to have substantially damaged his reputation among voters, given the gains made by the Tories in the Hartlepool byelection and councils across England. But some Conservatives are privately dismayed the party is being more opaque than they think it should be on issues of funding and declarations. The investigation into Johnson’s holiday comes after he took a trip to Mustique to celebrate new year at the start of 2020. The Daily Mail reported Johnson spent 10 days on a luxury villa break worth £15,000 – provided courtesy of the Carphone Warehouse founder and Conservative donor David Ross. Johnson did record a “benefit in kind” from Ross worth £15,000 for “accommodation for a private holiday for my partner and me” in St Vincent and the Grenadines from 26 December 2019 to 5 January 2020. It was submitted to the register of members’ interests on 27 January 2020. Questions were raised when the Daily Mail said a spokesperson for Ross initially said he had not paid for the trip and described the claim as a “mistake”, before backtracking and saying he had “facilitated” the trip. The standards commissioner said the section of the MPs’ code of conduct it was looking at in relation to allegations of impropriety said that: “Members shall fulfil conscientiously the requirements of the house in respect of the registration of interests in the register of members’ financial interests. They shall always be open and frank in drawing attention to any relevant interest in any proceeding of the house or its committees, and in any communications with ministers, members, public officials or public office holders.” The commissioner said the matter under investigation was “registration of interest under category 4 of the guide to the rules [visits outside the UK] in 2020”. Following the announcement of an investigation, a spokesperson 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.” Angela Rayner, Labour’s deputy leader, said: “The public have a right to know who paid for Boris Johnson’s luxury Caribbean holiday and the renovation of his flat. Most importantly, we need to know what these donors were promised or expected in return for their generosity. “As we have seen over the last year, Tory donors have received a very high return on their investment in the form of government contracts. Boris Johnson needs to stop using the office of prime minister as an opportunity to fund his lavish lifestyle and enrich his mates.” Downing Street said Johnson had “transparently declared” his break and “declared the benefit in kind in the Commons register of interests”. Asked if the government believed the public think the rules matter, the No 10 spokesperson said: “Clearly the rules are set out and it’s important that everyone abides by them, as the prime minister has done throughout.” It also emerged on Monday that the standards commissioner was refusing to launch an investigation into Johnson’s Downing Street flat payments due to the existing probe by the Electoral Commission. In a letter sent to Labour MP Margaret Hodge and seen by the Guardian, Stone said she had made no decision about whether to investigate Johnson’s actions “in his capacity as an MP” and would await the Electoral Commission’s report to “avoid potentially prejudicing” its own findings. Hodge, who was a tough scrutiniser in her former role as chair of the public accounts committee, told the Guardian that while she had a “great deal of respect” for Stone, she had “enormous concerns that we are tiptoeing around the problem”, raising concerns about how long the Electoral Commission’s inquiries could drag on. She added: “With an investigation into the Conservative party’s dealings under way and the Cabinet Office looking into the matter too, it is high time that the prime minister was investigated. This issue will not go away. Until there is a proper investigation, suspicions about the integrity and honesty of our prime minister will rumble on.”