Filed Pursuant to Rule 424(b)(3)

Registration No. 333-259982

PROSPECTUS SUPPLEMENT

(To Prospectus dated November 22, 2022)

Arqit Quantum Inc.

4,717,000 Ordinary Shares

6,266,667 Warrants to Purchase Ordinary Shares

595,666 Ordinary Shares Underlying Warrants

This prospectus supplement updates and amends certain information contained in the prospectus dated November 22, 2022 (the “Prospectus”) covering the offer and sale from time to time by the selling securityholders named in the Prospectus of up to 4,717,000 of our ordinary shares, par value $0.0025 per share (“Ordinary Shares”), and 6,266,667 warrants to purchase Ordinary Shares. The Prospectus also relates to the issuance by us of up to 595,666 Ordinary Shares, that are issuable by us upon the exercise of the Public Warrants (as defined in the Prospectus), which were previously registered, and the Private Warrants (as defined in the Prospectus). You should read this prospectus supplement in conjunction with the Prospectus. This prospectus supplement is not complete without, and may not be utilized except in connection with, the Prospectus, including any amendments or supplements thereto.

Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties described under the heading “Risk Factors” beginning on page 9 of the Prospectus, and under similar headings in any amendment or supplements to the Prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of the Prospectus or this prospectus supplement. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is December 9, 2025

EXPLANATORY NOTE

This prospectus supplement updates and supplements the Prospectus to update table of the Selling Securityholders (as defined in the Prospectus) to reflect certain sales and distributions that have been made as permitted under the section entitled “Plan of Distribution” included in the Prospectus. On September 25, 2024, Arqit Quantum Inc. effected a 25-to-1 reverse share split, which is reflected in the numbers included in this prospectus supplement.

Updated Table of Selling Securityholders

Where the name and information of a Selling Securityholder identified in the table below also appears in the table and the related footnotes in the Prospectus on pages 29, 30 and 31, the information set forth in the table below and the related footnotes regarding that Selling Securityholder supersedes and replaces the information regarding such Selling Securityholder in the Prospectus. The percentages in the following table are based on 22,320,108 ordinary shares outstanding, including (i) 15,651,859 ordinary shares issued and outstanding as of December 5, 2025, (ii) 6,582,734 ordinary shares underlying our currently exercisable outstanding warrants, and (iii) 85,515 shares underlying outstanding equity incentives that are exercisable or that will become exercisable within 60 days following December 5, 2025.