Prime Minister Rishi Sunak is being investigated by Parliament’s standards watchdog over a possible failure to declare an interest.
Mr Sunak is being investigated over whether a declaration of interest was “open and frank”, under rules set out by the commissioner for standards.
The BBC understands the probe relates to a childcare firm his wife has shares in.
The commissioner decides whether an MP has broken rules after an inquiry.
A Downing Street spokesperson said: “We are happy to assist the commissioner to clarify how this has been transparently declared as a ministerial interest.”
Last month, Mr Sunak faced questions over shares his wife, Akshata Murty, holds in Koru Kids, a childcare agency that could benefit from a new policy unveiled in the spring Budget.
Chancellor Jeremy Hunt announced a pilot of payments for new childminders, with more for those who sign up through agencies.
Ms Murty was listed as a shareholder in one of those agencies, Koru Kids, as recently as 6 March.
Mr Sunak did not mention Ms Murthy’s links to Koru Kids when he was questioned by MPs over the childcare policy at a parliamentary committee hearing on 28 March.
Labour MP Catherine McKinnell asked Mr Sunak whether he had any interest to declare, and in reply he said: “No, all my disclosures are declared in the normal way.”
In a letter to the committee, sent a few days after the hearing, Mr Sunak said his wife’s interest was declared to the Cabinet Office and that an updated statement of ministers’ interests would be due out shortly.
In his letter, Mr Sunak said the list of ministerial interests “ensures steps are taken to avoid or mitigate any potential conflict of interest”.
The list of ministerial interests is separate to the register of interests for MPs, which says members “must always consider whether they have a conflict of interest”.
The list has not been updated for nearly a year and was last compiled by Lord Geidt, who resigned as Boris Johnson’s ethics adviser.
Labour’s deputy leader Angela Rayner MP said the failure to update the rules or publish the register of ministers’ interests had “left a transparency black hole which is enabling the prime minister and those he has appointed to dodge proper scrutiny of their affairs”.
She added: “If Rishi Sunak has got nothing to hide, he should commit to publishing the register before May’s elections so the public can see for themselves.”
The Liberal Democrats said the investigation was another example of a Conservative prime minister allegedly “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,” Liberal Democrat chief whip Wendy Chamberlain said.
An update on the commissioner’s website says Mr Sunak is being investigated under paragraph 6 of code of conduct for MPs.
The paragraph reads: “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 commissioner for standards is an independent officer who investigates allegations that MPs have breached Parliament’s code of conduct.
Following investigation, if the watchdog thinks the allegation represents a breach of the code, they can put such cases before MPs sitting on the Committee on Standards, who can decide any sanctions.
Sanctions for breaching the code range from being ordered to apologise, to a suspension from Parliament in more serious cases.