Boris Johnson has been ordered to apologise to MPs for failing to declare more than £52,000 in income.
The former foreign secretary said he had “inadvertently” failed to register payments from book royalties in time.
But the Committee on Standards said he had “acted in breach of the House’s rules” and must apologise.
Parliament’s Standards Commissioner Kathryn Stone had earlier ruled that the breaches were “neither inadvertent nor minor”.
She referred the case to the Committee on Standards to decide on a punishment for Mr Johnson.
MPs are required to declare payments for any work carried out in addition to their duties as an MP, in the Register of Members’ Interests.
Ms Stone found that Mr Johnson’s registrations were late on four separate occasions, involving nine payments, “which suggested a lack of attention to, or regard for, the House’s requirements”.
“The committee concluded that Mr Johnson breached the rules of the House by failing to register remuneration within the required timetable on nine occasions,” the committee said in a report.
“In considering the appropriate sanction, it took into account what it considered to be aggravating and mitigating factors, and recommended that Mr Johnson make an apology to the House on a point of order.”
The late declarations included a series of rights and royalty payments from Bulgaria, Hungary, the Czech Republic, France and the US for books “already written” by Mr Johnson.
The committee said aggravating factors in calling for Mr Johnson to apologise included the size of the sums involved and the number of breaches.
As a longstanding MP who had been a senior minister, he “could be expected to set an example within the House,” it added.
Mr Johnson responded “promptly and helpfully” when the issue was raised to him, apologised to the Commissioner and put in place “effective measures to ensure that no further breach occurs”, the committee added.