Sir Keir Starmer urged the prime minister to publish his register of interests today after an investigation was opened into claims he may have broken parliamentary rules.

Mr Sunak is currently under investigation by the Commons' standards commissioner over claims he did not declare his wife's shareholding in a childcare agency when first asked by MPs.

Downing Street maintains that the prime minister did declare interest in the ministerial register and that it was "happy to assist the commissioner".

But Sir Keir said the investigation was "obviously serious" and called on Mr Sunak to "clear this up".

He pointed out that the register of ministerial interests had not been published for nearly a year.

Starmer calls on PM to 'come clean' and publish register of interests - politics latest

Speaking to broadcasters during a visit to York University, the Labour leader said: "It's obviously serious and transparency matters.

"The prime minister promised integrity and accountability when he came into office so I think he needs to clear this up and declare his interests.

"The register of ministerial interests hasn't been published for nearly a year, so I invite the prime minister to publish at least his entry on this issue today, clear this up, because he promised transparency."

He added: "Don't hide behind the process, just come clean and tell everybody what the interest is so people can see it and judge it. That's what transparency is."

Asked whether Mr Sunak would publish his register as a matter of urgency, the prime minister's official spokesperson indicated this could happen further down the line - but not today.

They said it was something Mr Sunak's independent adviser, Sir Laurie Magnus, was "looking at".

"I think he publicly said he wants to do [it] as quickly as possible, rather than it being a decision solely for the prime minister to make" the spokesman said.

"So I think that work is still ongoing, but obviously you have got the commitment from him to do it."

The investigation into Mr Sunak was launched last Thursday by parliament's Standards Commissioner Daniel Greenberg.

While Mr Greenberg did not go into the specifics of what Mr Sunak was being investigated for, he cited paragraph 6 of the MPs' code of conduct, which states that 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".

A Downing Street source confirmed the investigation related to shares held by Akshata Murty, Mr Sunak's wife, 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.

The allegation against Mr Sunak is that while he may have declared the interest in the unpublished ministerial interests list, he did not flag it when questioned by MPs on the liaison committee last month.

During the session, Labour MP Catherine McKinnell pointed out that six private childcare agencies were set to benefit from the budget, in which the chancellor announced 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.

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Ms McKinnell asked Mr Sunak what the logic was in making the bonus twice as much for childminders who sign up through private agencies.

He said the policy was "designed in consultation with the sector" and was a "reflection of the fact that they are through intermediaries so there are additional costs".

Pressed on whether he had any interest to declare, Mr Sunak replied: "No, all my disclosures are declared in the normal way."

The prime minister is just the latest MP to be investigated by Mr Greenberg, who is looking into the behaviour of six MPs in total.

Last week investigations were opened into three MPs, including former health secretary Matt Hancock, Tory MP Henry Smith and independent MP Scott Benton.