The Parliamentary Commissioner for Standards is investigating whether the Prime Minister broke the rules with his trip to the island of Mustique over the New Year in 2020.
It comes after Labour wrote to the commissioner calling for a full investigation into the Prime Minister’s register of interests.
On the Commons’ register of members’ interests, Mr Johnson declared the stay in a villa was a “benefit in kind”, with the donor listed as Mr David Ross, the Carphone Warehouse co-founder.
However, the businessman told the Daily Mail he had only helped put the Tory leader in touch with companies providing accommodation. He insisted he is not the owner of the villa and had not paid for the trip.
Number 10 previously insisted everything has been registered properly.
The commissioner for standards investigates allegations and complaints against MPs.