Rishi Sunak's wife is a shareholder in a childcare agency that will benefit from a major new policy announced in Chancellor Jeremy Hunt's Budget.
The Prime Minister failed to mention Akshata Murthy's links to Koru Kids when he was questioned by MPs over why the financial announcement favoured private firms.
But Companies House lists her as a shareholder in the firm as recently as March 6.
The Liberal Democrats said it raises "serious questions" for Mr Sunak and called for his ethics adviser, Sir Laurie Magnus, to investigate.
This month Mr Hunt 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.
Labour MP Catherine McKinnell quizzed Mr Sunak at the Liaison Committee on why the Budget was handing out a double bonus for childminders signing up through private agencies.
Asked yesterday by the chairwoman of the Petitions Committee if he had any interest to declare, Mr Sunak replied: "No, all my disclosures are declared in the normal way."
He added the policy was "designed in consultation with the sector".
Mr Sunak’s spokeswoman today said the rules had been followed “to the letter”.
She said he had declared any relevant interests, but that an updated version of the ministerial register was not due to be published until May.
“As the PM said yesterday, all interests have been declared in the usual way,” she added.
But Commons rules say even if they’ve registered interests formally, MPs are expected to “draw attention” to them in a situation where “someone else might consider them to influence what you say or do”.
The rules specifically include select committee appearances among those situations.
Koru Kids is one of six childminder agencies listed on the Government's website.
Welcoming the Budget, Koru Kids said on its website the "new incentives open to childminders are great".
It said a bonus of "£1,200 - yes double" would be paid "if you come through an agency like Koru Kids who offer community, training and ongoing support".
Mr Sunak and his wife last year entered The Sunday Times Rich List for the first time with their joint £730 million fortune.
Labour's Deputy Leader Angela Rayner said: “Rishi Sunak must explain why he failed to come clean when asked about the shares his family held in a company now set to financially benefit from a childcare policy announced in his Budget.
"He must urgently correct the record and set out what steps he took to avoid an actual or perceived conflict of interest. No proper explanation has yet been provided by the Prime Minister as to why this was not deemed necessary to publish in the register of members’ interests.”
In a letter to Sir Laurie the Lib Dems Chief Whip, Wendy Chamberlain wrote: “I urge you to investigate whether the Code has been broken; and if it has not, what steps the Prime Minister took to ensure that no conflict arose.
“In particular please set out whether he made you or other senior officials aware of the financial interest in Koru Kids when the childcare policy was being developed.”
In a separate statement, she added “There are serious questions for Rishi Sunak to answer over any potential conflict of interest, and any extra income his family could receive from his own government’s policy.
“Too often we have seen Conservative sleaze run amok. The public must be reassured that any breach of the ministerial code by the Prime Minister will be fully investigated.”
Sir Laurie does not have the power to open his own investigations without the PM’s permission.
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