Transaction in Own Shares

9 September 2026

Costain Group PLC

Transaction in own shares

Costain Group PLC ("Costain") announces that, in accordance with the terms of its share buyback programme (the "Programme") announced on 10 March 2026, it has purchased the following number of its ordinary shares of 1 pence each ("Ordinary Shares") on the London Stock Exchange through Investec in the period from 2 September 2026 to 8 September 2026.

The Company intends to cancel the purchased shares. The Company does not hold any Ordinary Shares in treasury.

Costain has now purchased 6,840,001 Ordinary Shares in aggregate for cancellation under the Buyback Programme. Following settlement of the above purchases and cancellation of the purchased Ordinary Shares, the Group's total number of Ordinary Shares in issue will be 262,916,034 Ordinary Shares.

Schedule of Purchases - Individual Transactions

In accordance with Article 5(1)(b) of Market Abuse Regulation (EU) No 596/2014 (as it forms part of the law of the United Kingdom by virtue of the European Union (Withdrawal) Act 2018, as implemented, retained, amended, extended, re-enacted or otherwise given effect in the United Kingdom from 1 January 2021 and as amended or supplemented in the United Kingdom thereafter) a full breakdown of the individual trades made by Investec on behalf of the Company as part of the Programme can be found at:

http://www.rns-pdf.londonstockexchange.com/rns/0620U_1-2026-9-9.pdf

The LEI reference for Costain Group PLC is 213800PKIJBZ2EDTKC88.

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