Sir Keir Starmer has denied wrongdoing after a Parliament investigation was launched into claims he twice broke the MPs' rules of conduct on registering interests.
The parliamentary website shows the Labour leader is being probed for one breach relating to the declaration of gifts and another on earnings.
A Labour source said the investigation centres on a number of slightly late declarations, which have been blamed on administrative errors.
During a visit to Wakefield, the Labour leader told broadcasters the allegations were not a surprise, adding: "My office is dealing with it and will be replying in due course."
Asked if he was sure he had done nothing wrong, he said: "Absolutely confident, there's no problem here."
Parliamentary Commissioner for Standards Kathryn Stone began her investigation on Wednesday last week, according to Parliament's website.
The first matter under investigation is stated as: "Registration of interests under Category 1 of the Guide to the Rules (Employment and earnings)."
And the second: "Registration of interests under Category 3 of the Guide to the Rules (Gifts, benefits and hospitality from UK sources)."
The nature of the alleged breaches is not yet clear.
A spokesperson for Keir Starmer said: "Keir Starmer takes his declaration responsibilities very seriously and has already apologised for the fact that administrative errors in his office have led to a small number of late declarations.
"The standards commissioner has asked for more information which we are happy to provide.”
The rules on employment and earnings say that MPs must register payments of more than £100 which they receive for work outside the Commons.
Under the section on gifts and hospitality, they must register benefits worth more than £300, or multiple benefits from the same source if they exceed that value in a calendar year.
The register shows that, as of the end of May, Sir Keir had registered earnings of £17,598.60 for legal advice given before 2020, the year in which he became the Opposition leader.
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It shows the lawyer received the sum on August 24 last year for around 70 hours of work, before registering it seven days later.
Two copyright payments for books written before his election to Parliament in the London constituency of Holborn and St Pancras are also included, as is a £18,450 advance from publisher HarperCollins for a book he is writing.
The matters under which he is being investigated relate to the section of the MPs' code which states: "Members shall fulfil conscientiously the requirements of the House in respect of the registration of interests in the Register of Members' Financial Interests.
"They shall always be open and frank in drawing attention to any relevant interest in any proceeding of the House or its committees, and in any communications with ministers, members, public officials or public office holders." The Labour leader is also being investigated by Durham Police over separate allegations after being accused of breaking coronavirus laws in April last year.