Sunak vowed "integrity, professionalism and accountability at every level" when he came to power, after the short-lived tenure of Liz Truss and the scandal-hit premiership of Boris Johnson.
But in a year in office, he has received a police fine for not wearing a seatbelt, on top of another for breaking Covid lockdown rules when he was finance minister under Johnson.
House of Commons Standards Commissioner Daniel Greenberg began a probe in April after receiving a complaint about Sunak's comments to a committee of senior MPs the previous month.
Sunak failed to mention to them and in a follow-up letter that his wife, Akshata Murthy, held shares in a childminding firm that would receive additional state funding.
He told Greenberg he had been advised that Murty's shareholding "did not meet the test of relevance to require publication on the List of Minister's Interests."
But the commissioner disagreed, saying that it "was a relevant interest that should have been declared" and he had "a duty to correct the record".
The omission may have arisen out of confusion about what was required and was "inadvertent", he concluded.
The prime minister's spokesman said the matter had been resolved. "The prime minister takes seriously his responsibilities to register and declare all relevant interests," he added.
During his time at the Treasury Sunak was criticised after it emerged that Murty, whose father co-founded Indian IT giant Infosys, had so-called "non-dom" status.
That meant she did not declare earnings from her dividends in the firm for UK tax purposes. Murty later said she would include them after a furore.
Critics, including the main opposition Labour party, have said Sunak, a wealthy former investment banker, is out of touch with ordinary voters struggling with rising costs.