The prime minister was already facing a probe by the standards commissioner into whether he failed to declare an interest over his wife's finances.

But an update on the commissioner's website on Monday showed he was also being looked into for another possible breach - this time over disclosing details about an ongoing investigation.

Politics live: Sunak faces new investigation into possible code of conduct breach

Standards commissioner Daniel Greenberg launched his inquiry into Mr Sunak earlier this month after it emerged the PM's wife, Akshata Murty, held shares in a childcare agency that could benefit from a policy announced in the last budget.

The investigation was looking into whether Mr Sunak breached paragraph 6 of the MPs' code of conduct, which states MPs "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".

Downing Street sources confirmed the probe was in relation to Ms Murty's shares in Koru Kids - one of the six childcare agencies listed on the government's website that welcomed Jeremy Hunt's announcement to offer cash incentives to childminders.

But a spokesperson insisted the interest had been "transparently declared", saying the PM would be "happy to assist" Mr Greenberg with his inquiry.

Days later, and after pressure from opposition parties, the delayed ministers' register of interest was published with a footnote showing Ms Murty's Koru Kids shares.

However, Number 10 continued to insist it had been a long-standing declaration by Mr Sunak, saying: "We have been very clear that the prime minister has taken his obligation to declare everything very seriously, he has done that for a number of years."

On Monday, Mr Greenberg extended his investigation to look into whether the PM breached paragraph 13 of the code of conduct.

It says: "Members must not disclose details in relation to: (i) any investigation by the Parliamentary Commissioner for Standards except when required by law to do so, or authorised by the Commissioner; nor (ii) the proceedings of the Committee on Standards or the Independent Expert Panel in relation to a complaint unless required by law to do so, or authorised by the Committee or the Panel respectively."

Asked about the new strand to the inquiry, the PM's official spokesman said they had only "pointed to what has been said previously" in public remarks.

Second Benton probe

The commissioner also confirmed on Monday that he had opened a second investigation into former Tory MP Scott Benton over whether he had undertaken "actions causing significant damage to the reputation of the House as a whole, or of its members generally".

Mr Benton had the Conservative whip removed - meaning he now sits as an independent MP - earlier this month after a sting operation by The Times newspaper secretly filmed him indicating he would be willing to break lobbying rules for money.

The commissioner announced an initial investigation into the Blackpool South MP on 11 April over whether he had breached paragraph 16 of the code of conduct, described as "use of facilities [parliamentary email address] provided from the public purse".