According to the Sunday Times, Starmer, in power since July, faces an investigation after "neglecting to disclose" that major Labour donor Waheed Alli covered the cost of a personal shopper, clothes and alterations for his wife.
The premier's registered financial interests, listed on the parliament's website, show that he has received several donations from Alli, including multiple pairs of glasses, work clothes and accommodation.
The Sunday Times said those donations were declared but the clothes given to his wife were not.
A spokesperson for 10 Downing Street said in a statement sent to Reuters that Starmer and his team had sought advice from authorities on coming to office, and believed they had been compliant.
"However, following further interrogation this month, we've declared further items," the spokesperson said on Saturday.
Alli is British media entrepreneur and former chairman of the online fashion retailer ASOS.
Under the House of Commons code of conduct, members of parliament have to provide information about financial interests which might reasonably be thought to influence their work.
A Conservative Party spokesperson called for a full investigation over "apparent serious breaches of parliamentary rules".
