Costain Regulatory News (COST)

For filings with the FCA include the annex

For filings with issuer exclude the annex

TR-1: NOTIFICATION OF MAJOR INTEREST IN SHARESi

1. Identity of the issuer or the underlying issuerof existing shares to which voting rights areattached: ii

Costain Group PLC

2 Reason for the notification (please tick the appropriate box or boxes):

An acquisition or disposal of voting rights

An acquisition or disposal of qualifying financial instruments which may result in the acquisition of shares already issued to which voting rights are attached

An acquisition or disposal of instruments with similar economic effect to qualifying financial instruments

An event changing the breakdown of voting rights

Other (please specify):

3. Full name of person(s) subject to the notification obligation: iii

BlackRock, Inc.

4. Full name of shareholder(s) (if different from 3.):iv

5. Date of the transaction and date on which the threshold is crossed or reached: v

15 September, 2016

6. Date on which issuer notified:

16 September, 2016

7. Threshold(s) that is/are crossed or reached: vi, vii

Voting rights attached to shares holding for BlackRock, Inc. has gone below 5%

8. Notified details:

A: Voting rights attached to shares viii, ix

Class/type ofshares

if possible usingthe ISIN CODE

Situation previousto the triggeringtransaction

Resulting situation after the triggering transaction

NumberofShares

NumberofVotingRights

Numberof shares

Number of votingrights

% of voting rights x

Direct

Direct xi

Indirect xii

Direct

Indirect

GB00B64NSP76

5,178,395

5,178,395

N/A

N/A

4,120,850

N/A

4.00%

B: Qualifying Financial Instruments

Resulting situation after the triggering transaction

Type of financialinstrument

Expirationdate xiii

Exercise/Conversion Period xiv

Number of votingrights that may beacquired if theinstrument isexercised/ converted.

% of votingrights

Securities Lending

1,118,974

1.08%

C: Financial Instruments with similar economic effect to Qualifying Financial Instruments xv, xvi

Resulting situation after the triggering transaction

Type of financialinstrument

Exercise price

Expiration date xvii

Exercise/Conversion period xviii

Number of voting rights instrument refers to

% of voting rights xix, xx

CFD

640,858

Nominal

Delta

0.62%

0.62%

Total (A+B+C)

Number of voting rights

Percentage of voting rights

5,880,682

5.71%

9. Chain of controlled undertakings through which the voting rights and/or thefinancial instruments are effectively held, if applicable: xxi

See Annex 1

Proxy Voting:

10. Name of the proxy holder:

11. Number of voting rights proxy holder will ceaseto hold:

12. Date on which proxy holder will cease to holdvoting rights:

13. Additional information:

BlackRock Regulatory Threshold Reporting Team

14. Contact name:

Marc-Oliver Lesch

15. Contact telephone number:

020 7743 3650

Annex 1

Name

% of voting rights if it equals or is higher than the notifiable threshold

% of voting rights through financial instruments if it equals or is higher than the notifiable threshold

Total of both if it equals or is higher than the notifiable threshold

BlackRock, Inc.

BlackRock Holdco 2, Inc.

BlackRock Financial Management, Inc.

BlackRock Holdco 4, LLC

BlackRock Holdco 6, LLC

BlackRock Delaware Holdings Inc.

BlackRock Fund Advisors

BlackRock Institutional Trust Company, National Association

BlackRock, Inc.

BlackRock Holdco 2, Inc.

BlackRock Financial Management, Inc.

BlackRock International Holdings, Inc.

BR Jersey International Holdings L.P.

BlackRock Group Limited

BlackRock Advisors (UK) Limited

BlackRock, Inc.

BlackRock Holdco 2, Inc.

BlackRock Financial Management, Inc.

BlackRock International Holdings, Inc.

BR Jersey International Holdings L.P.

BlackRock Group Limited

BlackRock Investment Management (UK) Limited

BlackRock, Inc.

BlackRock Holdco 2, Inc.

BlackRock Financial Management, Inc.

BlackRock Holdco 4, LLC

BlackRock Holdco 6, LLC

BlackRock Delaware Holdings Inc.

BlackRock Fund Advisors