Boris Johnson has apologised in parliament over the late declaration of more than £52,000 of outside earnings.

The ex-foreign secretary was rebuked by the Committee on Standards for breaching strict rules to inform the Commons authorities within 28 days about payments received outside of his work as an MP.

Parliamentary Commissioner for Standards, Kathryn Stone, had previously found the breach was “neither inadvertent nor minor” due to the amount of money and Mr Johnson’s failure to meet the deadline regarding nine payments.

Most of the declarations relate to royalties from Mr Johnson’s books, which cover subjects from Sir Winston Churchill to the Roman Empire. He has already accepted that he accidentally broke the rules.

Ms Stone said: “Although Mr Johnson has told me that the late registrations were ‘inadvertent’, the fact that the late registrations had happened on four separate occasions and involved nine payments, suggests a lack of attention to, or regard for, the House’s requirements rather than oversight or inadvertent error.”

She added: “The total amount of money involved represented a significant proportion of the annual parliamentary salary.

“On the other hand, the individual payments were made irregularly and, as Mr Johnson has pointed out, their timing was not closely linked to recent hours of work. Having made five late registrations in December 2017, Mr Johnson did not take steps to avoid a recurrence until I initiated an inquiry in October 2018.

“The steps Mr Johnson has since taken appear sufficient to avoid a recurrence and he has expressed a willingness to adopt additional measures if they prove necessary. Mr Johnson apologised as soon as this matter was drawn to his attention and he has co-operated fully and promptly with my enquiries.”

After the commissioner passed on her findings to the cross-party Committee on Standards to come up with appropriate sanctions, the leading Brexiteer was subsequently ordered to make an apology in the Commons over the blunder.

The committee said that Mr Johnson had not “intended to deceive the House or the general public about the level of his remuneration”, while criticising his “over-casual attitude” to the rules.

Its report said: “The committee concluded that Mr Johnson breached the rules of the House by failing to register remuneration within the required timetable on nine occasions.

“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.”

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Making his apology, Mr Johnson told MPs: “The Committee on Standards has today published a report on nine payments, mainly unexpected foreign royalties, which I am very sorry to say were recorded late on the register of Members’ interests.

“I fully accept that the delay was a breach of the House’s rules and, though I’m grateful to the committee for recognising that there was no intention to mislead the House and that I had been completely transparent, I therefore offer the House a full and unreserved apology.”

Mr Johnson has previously come under fire for failing to ask the ministerial watchdog for permission to take on a job writing a lucrative column for The Daily Telegraph.