Costain Group Regulatory News (COST)

10 March 2026

On-market Share Buyback Programme of £20m

Costain Group PLC ("Costain" or the "Group") today announces the launch of an on-market share buyback programme (the "Buyback") for a maximum aggregate consideration of £20m.

This follows the intention set out in the announcement dated 26 January 2026, which provided an update on the Group's new pension scheme agreement, enhanced shareholder returns and FY 25 trading. The removal of the dividend parity arrangement, as announced in the update, removed a significant constraint that was on Costain in respect of returns to shareholders.

Having considered the new agreement, the Group's cash performance and ongoing capital requirements, the Board has concluded that an on-market share buyback programme for a maximum aggregate consideration of £20m (excluding stamp duty and expenses) is an appropriate and value-enhancing use of cash, while maintaining the Group's financial flexibility to continue to invest in its strategy to deliver sustainable growth and attractive returns.

Any ordinary shares purchased by the Company will be cancelled and, accordingly, the Buyback will reduce the Company's share capital.

Details of the Buyback

Costain has instructed Panmure Liberum Limited ("Panmure Liberum") and Investec Bank plc ("Investec") to execute the Buyback. Costain has entered into a non-discretionary and irrevocable instruction with Panmure Liberum and Investec, pursuant to which Panmure Liberum (in respect of the First Tranche) and Investec (in respect of the Second Tranche) will each purchase the Company's ordinary shares of 1 pence each ("Ordinary Shares") for up to a maximum consideration of £10m for each tranche, each acting severally as riskless principal. The First Tranche will commence immediately. The Second Tranche will commence following completion of the First Tranche and it is anticipated it will end no later than 31 December 2026, subject to market conditions.

The maximum number of Ordinary Shares that can be purchased in aggregate under the First Tranche and Second Tranche is 20,481,508, being the number of shares the Company is authorised to purchase pursuant to the authority granted by shareholders at the Company's 2025 annual general meeting (the "2025 AGM") and any subsequent authority.

Panmure Liberum and Investec will make their trading decisions in relation to the Ordinary Shares independently of, and uninfluenced by, the Company, within the terms and pre-set parameters of the Buyback.

Any purchase of Ordinary Shares under the Buyback will take place in open market transactions and may be made from time to time depending on market conditions, share price and trading volumes. The Buyback will be effected under the general authority to repurchase Ordinary Shares granted by the Company's shareholders at the 2025 AGM and in accordance with Chapter 12 of the UK Financial Conduct Authority's Listing Rules, and Regulation (EU) No 596/2014 and Commission Delegated Regulation (EU) No 2016/1052 (both as they form part of UK domestic law by virtue of the European Union (Withdrawal) Act 2018), including where relevant pursuant to the UK Market Abuse Regulation.

Any purchase of Ordinary Shares pursuant to the Buyback will be announced by no later the end of the 7th daily market session following the calendar day on which such transaction occurred.

The Company will make further announcements in due course following the completion of any repurchases. There is no guarantee that the Buyback will be implemented in full or that any Ordinary Shares will be repurchased by the Company. The Buyback will not impact the Company's existing Ordinary Share dividend policy, which will continue unaffected through the regular awards of interim and final dividends. At the time of this announcement, the Company's share capital comprises 266,714,895 Ordinary Shares with voting rights.

Enquiries:

Investors and analysts matt.jones@costain.com

Matt Jones, Costain +44 (0) 7860 922341

Financial media - Headland costain@headlandconsultancy.com

Andy Rivett-Carnac +44 (0) 7968 997 365

Charlie Twigg +44 (0) 7946 494 568

Notes to editors

Costain improves people's lives by creating connected, sustainable infrastructure that enables people and the planet to thrive. Through the delivery of predictable, best-in-class solutions across the transport, water, energy and defence markets, we are creating a sustainable future and securing a more prosperous, resilient and decarbonised UK.