1 April 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 Panmure Liberum in the period from 25 March 2026 to 31 March 2026.

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

Costain has now purchased 813,559 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 265,901,336 Ordinary Shares.

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 Panmure Liberum on behalf of the Company as part of the Programme can be found at:

http://www.rns-pdf.londonstockexchange.com/rns/0334Z_1-2026-4-1.pdf

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