Rishi Sunak is under investigation by the parliamentary standards commissioner, the watchdog has announced.
An inquiry has been launched into the prime minister’s “declaration of an interest”.
It is understood to be in relation to his multi-millionaire wife’s shares in a childcare company.
A No.10 spokesperson said: “We are happy to assist the commissioner to clarify how this has been transparently declared as a ministerial interest.”
Last month it was revealed by the i paper that Sunak’s wife, Akshata Murthy, is a shareholder in Koru Kids, which looks set to benefit from the government’s new childcare policy.
But the prime minister did not mention Murthy’s links the company when he was questioned by MPs over why it favoured private firms.
The government has launched a pilot of incentive payments of £600 for childminders joining the profession, a sum that doubles to £1,200 if they sign up through an agency.
Koru Kids is one of six childminder agencies listed on the government’s website.
An update to the parliamentary website shows the investigation into Sunak, opened on Thursday, relates to paragraph six of the code of conduct.
“Members must always be open and frank in declaring any relevant interest in any proceeding of the House or its committees, and in any communications with ministers, members, public officials or public office holders,” the section reads.
Deputy Labour leader Angela Rayner said: “This government’s failure to update the rules or publish a register of ministers’ interests in nearly a year has left a transparency black hole which is enabling the prime minister and those he has appointed to dodge proper scrutiny of their affairs.
Lib Dem chief whip Wendy Chamberlain said: “Another day and another accusation of a Conservative prime minister bending the rules.
“After months of Conservative sleaze and scandal, the public just want a government which is focused on the country, rather than saving their own skin.”
Last week it was revealed the other MPs, including former health secretary Matt Hancock, were also under investigation by the commissioner.