On-market Share Buyback Programme of £10m

Costain Group PLC ("Costain" or the "Group") today announces its intention to launch an on-market share buyback programme for up to a maximum aggregate consideration of £10m (excluding stamp duty and expenses) (the "Buyback"), to be carried out in two tranches. The first tranche of up to a maximum consideration of £5m (the "First Tranche") will commence immediately. The second tranche of up to a maximum consideration of £5m (the "Second Tranche") will commence following completion of the First Tranche and it is anticipated it will end no later than 28 March 2025, subject to market conditions.

The purpose of the Buyback is to reduce the Company's share capital and, accordingly, any ordinary shares purchased by the Company will be cancelled. The Buyback will also return surplus capital to shareholders and increase earnings per share, while maintaining the financial flexibility to invest in the Group's strategy to confidently deliver sustainable growth and attractive returns.

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 £5m 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 28 March 2025, subject to market conditions.

The maximum number of Ordinary Shares that can be purchased in aggregate under the First Tranche and Second Tranche is 27,671,888, being the number of shares the Company is authorised to purchase pursuant to the authority granted by shareholders at the Company's 2024 annual general meeting (the "2024 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 2024 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 not later than 07:30 on the business day following the calendar day on which the purchase 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 278,466,029 Ordinary Shares with voting rights.