British Prime Minister Boris Johnson is being investigated over a luxury holiday he took in the Caribbean in 2019 following his election victory.
The UK Parliamentary Commissioner for Standards named Johnson on a list of allegations currently under investigation on its website, Monday.
Under the category “Matter under investigation” Johnson’s name appears next the statement, “Registration of interest under Category 4 or the Guide to the rules [Visits outside the UK] in 2020."
The Conservative leader and his fiancée Carrie Symonds spent New Year's Eve on the private island of Mustique in the Caribbean archipelago of the Grenadines.
In his declaration of interests as an MP, Johnson said the €17,400 holiday was given to him by businessman David Ross, a Conservative Party donor and founder of the Carphone Warehouse retail chain.
But Mr Ross caused confusion by initially denying that he had given him such a sum and then retracting his statement. But a spokesman insisted it was a "benefit in kind".
The rule says that politicians must register all visits outside the UK if they have not been paid for personally or by the state.
Downing Street had previously stressed that everything had been properly declared.
If the investigation finds Johnson to be in breach of the rules, he could be forced to apologise, or, if the commissioner determines the breach is serious enough, it may be referred to a committee that has powers to recommend further sanctions including a temporary suspension from parliament and even expulsion; although the latter is very rare.
The revelation comes as Boris Johnson is embroiled in a series of scandals that have highlighted the close links between the government and private interests, including the lavish renovation of his Downing Street flat, which is at the centre of an investigation by the UK Electoral Commission.
Despite these scandals, his Conservative Party emerged stronger from the 6 May local elections in England, taking Hartlepool, one of its historic strongholds in the north-east of the country, from Labour.