LONDON - Parliament's standards watchdog is investigating British Prime Minister Boris Johnson over how he reported his private foreign travel, although Johnson's spokesman said a Caribbean holiday provided by a donor was correctly registered with the authorities.

The Parliamentary Commissioner for Standards named Johnson in a newly-published list of ongoing investigations, saying it was looking at whether he had broken the code of conduct for members of parliament.

The inquiry increases scrutiny of Johnson's finances - with electoral authorities separately looking at how the refurbishment of his apartment was funded. The opposition has accused his government of being riddled with sleaze and cronyism. 

Johnson declared to parliament in 2020 that he had travelled to the Caribbean for a holiday with his partner, Carrie Symonds, from Dec. 26 2019 to Jan. 5 2020. The register showed 15,000 pounds worth of accommodation was provided by David Ross, co-founder of the Carphone Warehouse retail chain.

"The prime minister transparently declared the benefit in kind in the Common's register of interests," the spokesman said when asked about that holiday. "The prime minister's declaration is correct."

Monday's release said Johnson was being investigated under the section of the rules relating to travel outside Britain, and referred to a section which said lawmakers must be open and frank in disclosing relevant interests.

The release did not specify the exact nature of the investigation, nor which trip was being investigated.

The rule book states lawmakers must register all visits outside Britain if they have not been paid for personally or by the state.

The opposition Labour Party said Johnson needed to explain the arrangements around the Caribbean holiday further.

If found in breach of the rules, Johnson could be forced to apologise, or, if the commissioner determines the breach is serious enough, it could be referred to a committee that has powers to recommend further sanctions which range from temporary suspension from parliament to expulsion.

Cases of expulsion are extremely rare.

(Reporting by William James, editing by Elizabeth Piper and Angus MacSwan) ((william.james@thomsonreuters.com; +44 207 542 3374; Reuters Messaging: william.james.thomsonreuters.com@reuters.net))