The parliamentary standards commissioner has confirmed the prime minister is being investigated over his 2019 Caribbean holiday.
Independent commissioner Kathryn Stone revealed on Monday that Boris Johnson was included in a list of MPs being investigated for breaking the code of conduct.
The watchdog will probe who paid for the PM's stay in a villa in Mustique in Christmas 2019.
The ten-day trip, which Johnson took with his fiancée Carrie Symonds, reportedly cost around £15,000.
The PM claimed at the time the cost of the villa had been paid by Tory party donor David Ross, who owns property on the island.
Ross later denied having paid for the trip and insisted he did not own the villa in which Johnson and Symonds had stayed, instead claiming he had helped facilitate the trip, but did not reveal who had funded the getaway.
Stone revealed she was investigating the PM over whether he had breached paragraph 14 of the code of conduct for MPs, which states MPs should "fulfil conscientiously the requirements of the House in respect of the registration of interests."
The new list published by the standards watchdog names eight other MPs, including senior Conservative Owen Paterson, who is being investigated over four alleged breaches of the code, including around paid advocacy, declaration of interests, and the "use of facilities provided from the public purse".
Several other Conservative MPs are also being investigated for their use of House of Commons stationary.
Stone said she planned to published the list to "encourage greater confidence in the parliamentary standards system".
Her powers include the ability to suspend MPs from the Commons, or even force byelections if a breach is found to be sufficiently serious.
The announcement will put further pressure on the prime minister, who is also facing several probes into how he paid for recent renovations to the Downing Street flat.
John Johnston is a reporter for CSW's sister title PoliticsHome, where this story first appeared.