The British parliament's Commissioner for Standards opened an investigation into Prime Minister Rishi Sunak earlier this month, according to a list of open inquiries posted on its website on Monday.
The list said the matter under investigation was a "declaration of interest".
Sunak's office did not have an immediate comment.
The Commissioner is responsible for monitoring the operation of the House of Commons Code of Conduct and Registers, including investigating any alleged breaches. The commissioner's page on the parliament website did not provide further details on the possible breach.
Under the code of conduct all members of parliament are required to "to provide information about any financial interest which might reasonably be thought by others to influence a Member's actions, speeches or votes in Parliament, or actions taken in his or her capacity as a Member of Parliament."