Costain Group PLC

30 March 2022

Notice of 2022 Annual General Meeting and

Annual Report and Accounts for the year ended 31 December 2021

The Company announces that its Annual General Meeting will be held on Thursday 5 May 2022 at 3.00pm at No. 11 Cavendish Square, London, W1G 0AN (the "2022 Annual General Meeting").

Notice of the 2022 Annual General Meeting (the "2022 AGM Notice") and a form of proxy, together with the Annual Report and Accounts for the year ended 31 December 2021 (the "2021 Annual Report"), will be posted today to those shareholders who have elected to receive paper copies.

The 2022 AGM Notice and 2021 Annual Report are also available on the Company's website at www.costain.com and have been submitted to the National Storage Mechanism, where they will shortly be available for inspection at https://data.fca.org.uk/#/nsm/nationalstoragemechanism.

Protecting the health and wellbeing of our shareholders, colleagues and other stakeholders is a priority for the Costain Board of Directors. Accordingly, shareholders are requested not to attend the meeting in person if they display any symptoms of COVID-19 or have recently been in contact with anyone who has tested positive for COVID-19. In order to further reduce the risks posed by COVID-19, we are encouraging all attendees to take a lateral flow test before attending the 2022 AGM and to only attend if the result is negative.

Shareholders will be able to watch the AGM via a live webcast. For details of how to register for the webcast, please visit our website at www.costain.com/agm from 28 April 2022. Details required to register can be found in the 2022 AGM Notice. If you are unable to observe the webcast live, it will be available to view on-demand after the AGM.

The Board is also keen to ensure that shareholders are able to exercise their right to participate in the meeting by voting. Details on how to submit a proxy vote by post, online or through CREST are set out in the 2022 AGM Notice.

The Board recognises the importance for our shareholders to make an informed vote and ask questions. Registered shareholders may therefore submit questions to the Directors in advance of the meeting by sending an email to AGM@costain.com. The Company will answer these in due course including, where possible and appropriate, by publishing the response on the website at www.costain.com/agm before the proxy voting deadline or by answering the relevant question at the meeting.

Any changes to the AGM will be communicated to shareholders before the meeting through our website at www.costain.com/agm and, where appropriate, by RIS announcement.

Enquiries

Investors and analysts

Louise Bryant, Costain                                                            +44 7813 210 809

Financial media

Tim Rowntree, MHP                                                               +44 203 128 8771

Peter Hewer, MHP                                                                 Costain@mhpc.com

LEI: 213800PKIJBZ2EDTKC88