Sir Keir Starmer may have broken parliamentary rules in failing to declare clothes bought for his wife by Labour donor Lord Waheed Alli.
According to the Sunday Times, the donations also covered the cost of a personal shopper and alterations for Victoria, both before and after the general election.
The prime minister reportedly approached parliamentary authorities on Tuesday to make a late declaration after being given fresh advice on what items needed to be disclosed.
A No 10 spokesperson said Sir Keir believed he had complied with the rules, but had since declared further items.
MPs are required to register gifts and donations within 28 days of receiving them.
A No 10 spokeswoman 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."
Lord Alli has previously bought clothes and glasses for Sir Keir, which have been listed on his entry on the register of MPs' interests.
Most recently, the prime minister declared the fact Lord Alli had provided him with accommodation for several weeks, which is recorded as being worth more than £20,000.
In August, it emerged Lord Alli had been given a temporary Downing Street security pass despite having no formal government role.
The Conservative Party has called for the prime minister to face a "full investigation" into the "apparent serious breaches of parliamentary rules".
Andy Carter, a former Conservative MP who sat on Parliament's Standards Committee, told the BBC that registering interests was a "base level responsibility of MPs" and that Sir Keir "has form on this".
The prime minister was referred to the Parliamentary Standards Commissioner in 2022, when he was found to have been late in registering eight interests, including tickets to football matches and the sale of a plot of land.
The Parliamentary Standards Commissioner has not yet commented on the Sunday Times report.