By Ian Walker

LONDON--Water and recycling firm Pennon Group PLC (PNN.LN) is expected to return to the top flight when trading starts Friday, replacing power firm International Power PLC (IPR.LN) when its takeover by GDF Suez SA (GSZ.FR) completes Thursday, according to FTSE Group data Wednesday.

Pennon, which owns South West Water Ltd. and Viridor Ltd., was a member of the U.K.'s leading FTSE100 share index between Jan. 9, 2009, and Sept. 9, 2009.

International Power, which was already 70% owned by GDF Suez, finally accepted a 418 pence a share offer by GDF Suez for the remaining 30% it didn't own on April 16, valuing the U.K. firm at 22.8 billion pounds ($35.57 billion), after rejecting an earlier 390 pence offer. Under the new offer, shareholders were also able to keep the final dividend of 6.6 euro cents per share.

The merger is subject to court sanction, which is expected to take place on June 28 allowing the merger to complete June 29.

In the latest quarterly review in June, U.K. hedge fund manager Man Group PLC (EMG.LN) dropped out of the index and was replaced by engineering company Babcock International PLC (BAB.LN).

The FTSE100 is a share index of the 100 most highly capitalized companies listed on the London Stock Exchange.

Pennon shares at 0945 GMT were up 8 pence, or 1.05%, at 768.5 pence in a slightly lower FTSE250 Index--down 0.12%. International Power shares were unchanged at 417.4 pence in a slightly higher FTSE100 index--up 0.24%.

Write to Ian Walker at ian.walker@dowjones.com