Rishi Sunak's wife is a shareholder in a childcare agency that will benefit from a major new policy announced in the spring budget.
The prime minister did not mention Akshata Murthy's links to Koru Kids when he was questioned by MPs over why the financial announcement favoured private firms.
But as first reported by the i newspaper, Companies House lists her as a shareholder in the organisation as recently as 6 March.
The Liberal Democrats said this raises "serious questions" for Mr Sunak and called for his ethics adviser, Sir Laurie Magnus, to investigate.
This month Chancellor Jeremy 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.
At parliament's Liaison Committee on Tuesday, Labour MP Catherine McKinnell quizzed Mr Sunak on the logic behind making the bonus twice as much for childminders who sign up through private agencies.
