Keir Starmer is alleged to have broken parliamentary rules by failing to declare donations of clothing for his wife from the Labour donor Waheed Alli.
The gifts to Victoria Starmer were not initially declared in the register of MPs’ interests, the Sunday Times reported.
Starmer approached the parliamentary authorities on Tuesday to make a late declaration after being given updated advice on what needed to be registered.
The donations reportedly covered the cost of a personal shopper, clothes and alterations for Lady Starmer before and after Labour’s election win in July.
MPs are required to register gifts and donations within 28 days.
The Tories have demanded a full investigation into the Starmers’ links with Lord Alli, who has donated £500,000 to Labour since 2020.
A No 10 spokesperson said: “We sought advice from the authorities on coming to office.
“We believed we had been compliant, however, following further interrogation this month, we have declared further items.”
The guide to the Commons rules states MPs should register “any benefit given to any third party, whether or not this accompanied a benefit for him or her, if the Member is aware, or could reasonably be expected to be aware, of the benefit and that it was given because of his or her membership of the House or parliamentary or political activities”.
Last weekend it emerged that Alli had been given a Downing Street security pass temporarily without apparently having a government role.
The row was dubbed the “passes for glasses” affair because the television mogul had previously donated tens of thousands of pounds worth of clothing, accommodation and “multiple pairs” of spectacles to the Labour leader. There is no suggestion that the peer has broken any rules.
Alli, 59, was the youngest member of the House of Lords when he was ennobled in 1998.
A Conservative party spokesperson said: “It’s taken just 10 weeks for Keir Starmer to face an investigation for his conduct.
“After facing allegations of cronyism and now apparent serious breaches of parliamentary rules there must be a full investigation into the passes for glasses scandal.
“No doubt the millions of vulnerable pensioners across the country who face choosing between heating and eating would jump at the chance for free clothes just to keep warm in the face of Labour’s cruel cut.”