LONDON (Reuters) - U.S. activist investor Elliott Capital Advisors disclosed on Monday it has taken a 6.8 percent stake in WS Atkins (>> WS Atkins PLC) after the British engineering and construction consultancy firm agreed to be bought in a C$3.6 billion (£2 billion) deal.

Elliott said in a regulatory filing that it acquired the stake in a contracts-for-difference deal on April 21, the day after Atkins agreed to be acquired by Canadian construction and engineering group SNC Lavalin (>> Snc-Lavalin Group Inc).

Shares in Atkins closed on Monday at 2,100 pence, above SNC's offer price of 2,080 pence a share.

After SNC made an indicative offer for Atkins on April 3, Liberum analyst Joe Brent said in a note that the move could spark a bidding war, given growing consolidation in the industry. He declined to comment on Monday.

The stake in Atkins opens up another front in Europe for Elliott, as it presses Dutch-based paintmaker Akzo Nobel (>> Akzo Nobel) to open takeover talks with U.S. suitor PPG Industries (>> PPG Industries, Inc.), and separately campaigns for a restructuring at Anglo-Australian miner BHP Billiton (>> BHP Billiton plc) (>> BHP Billiton Limited).

Elliott's stake in Atkins makes it the second-biggest shareholder, Thomson Reuters data showed, just behind asset manager Columbia Threadneedle.

A spokeswoman for Elliott declined to comment on why the firm had taken a stake in Atkins. Spokesmen for Atkins and for SNC also declined to comment.

(Additional reporting by Justin Varghese and Esha Vaish in Bangalore and Allison Lampert in Montreal; Editing by Greg Mahlich)

By Vikram Subhedar and Simon Jessop