By Ian Walker

LONDON--Water and waste management firm Pennon Group PLC (PNN.LN) is due to be demoted from the U.K.'s leading FTSE100 share index next week after only two quarters, and will be replaced by holiday firm TUI Travel PLC (TT.LN), according to indicative data supplied by FTSE Group and based on last night's closing share prices.

Pennon joined the FTSE100 back in June, replacing International Power which was taken over by GDF Suez SA (GSZ.FR). Pennon, which owns South West Water Ltd. and Viridor Ltd., was last a member of the index between Jan. 9, 2009, and Sept. 9, 2009.

TUI Travel, which trades through a number of brands including airtours, Hayes & Jarvis and Thomson, has seen its share price rise 79% over the past six months from lows of 157 pence in June. Last week it posted a 39% rise in fiscal 2012 pretax profit to 201 million pounds ($322.52 million) after a strong summer booking period and said this has continued into Winter 2012/13.

The official announcement of changes, which is subject to the approval of the FTSE EMEA Committee, will be released Wednesday, based on tonight's closing share prices.

Changes to the index will be effective from Dec. 21.

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

Any company that falls to 111th and below is automatically ejected from the top-flight index, while any firm that rises to 90 or above is automatically promoted.

Pennon shares at 1123 GMT up 8 pence, or 1.32% at 616 pence. TUI shares up 1 pence, or 0.21%, at 285 pence. FTSE100 index up 0.25%; FTSE250 index up 0.2%.

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

Subscribe to WSJ: http://online.wsj.com?mod=djnwires