Boris Johnson is facing yet another investigation into whether donations he received were properly declared. The parliamentary standards commissioner, Kathryn Stone, has announced she is investigating a possible breach of the MPs’ code of conduct over the funding of a 10-day holiday the prime minister and his fiancee, Carrie Symonds, took to Mustique last year. What do we know? It is a matter of public record that Johnson got a £15,000 benefit-in-kind from David Ross, a Tory donor, for “accommodation for a private holiday” on a trip to St Vincent and the Grenadines from 26 December 2019 to 5 January 2020. That was declared by the prime minister in the register of MPs’ financial interests and submitted a few weeks after his trip home – on 27 January. Why is that controversial? On the surface, it is an open-and-shut case of an MP receiving a donation and registering it fairly swiftly, as he should have done. However, eyebrows were raised when the Daily Mail reported that Ross’s spokesman initially said the declaration was a “mistake”, and then later said he only “facilitated” the accommodation. The newspaper said the spokesman told them Johnson did not stay in Ross’s house, but that when the prime minister wanted to find somewhere to stay in Mustique, Ross called the company that ran all the villas on the islands and found one where a prospective visitor had dropped out. That raised questions about who did pay for the rental costs or whether the fee was waived. A spokesman for Ross, who was forced to quit as the deputy chair of Carphone Warehouse in 2008 after failing to disclose that he had pledged a large proportion of his stake in the company against personal loans, said his actions constituted a benefit-in-kind and so the declaration Johnson made to the Commons was “correct”. The Daily Mail also reported that the American owner of the villa, Sarah Richardson, said that use of it did not come free, and that she and her husband, Craig, had rented it out and “got paid” – but was not sure by whom. Has Johnson failed to declare something he should have? Downing Street insists the prime minister followed all transparency requirements. But the parliamentary commissioner for standards, who acts as a watchdog for MPs’ behaviour, clearly felt the need to dig deeper. It was finally confirmed on Monday that she was conducting an investigation into Johnson’s declarations. What rules may have been broken? The commissioner said the matter for which she was investigating Johnson specifically related to the rules on declaring financial interests concerning visits outside the UK. She said the MPs’ code of conduct states they should “fulfil conscientiously the requirements of the house in respect of the registration of interests in the register of members’ financial interests” and “always be open and frank in drawing attention to any relevant interest”. Financial interests are meant to be recorded by all MPs so anyone who has donated, loaned or given them money is identified, to ensure people cannot be currying favour under the radar. How many sleaze inquiries is Johnson facing? The Electoral Commission is carrying out its own investigation into allegations that donations to cover the cost of Johnson and Symonds’ Downing Street flat makeover were not properly declared. The commission said there were “reasonable grounds to suspect that an offence or offences may have occurred”. Simon Case, the cabinet secretary, is also conducting a review of the issue. The parliamentary standards commissioner has been encouraged to open a separate investigation into the flat payments issue. Downing Street has said Johnson paid the sums – reported to run to £200,000 – “personally”, but has not denied that a loan was initially used, either from another Tory donor or from the party itself, to help the prime minister foot the bill.