UPDATE 1-Aggreko to enter FTSE 100, Rentokil to leave

* Aggreko will enter the FTSE 100 index on Dec. 21

* Rentokil Initial slips to FTSE 250 .FTMC

* Four companies to be promoted to mid-cap index

By Harpreet Bhal

LONDON, Dec 9 (Reuters) - Temporary power producer Aggreko (AGGK.L) will enter the FTSE 100 .FTSE later this month, replacing pest control-to-parcel delivery firm Rentokil Initial (RTO.L) after a quarterly review, index complier FTSE said on Wednesday.

Rentokil will be demoted to the mid-cap stocks .FTMC, having spent just one quarter in the index of the top 100 companies by market capitalisation. The changes will take effect on Dec. 21, FTSE said.

Companies outside the FTSE 100 that grow to rank among the 90 largest by market capitalisation are automatically promoted into the blue-chip index, while the FTSE 100 firms with the lowest value, or that fall to 111th spot or below, drop into the FTSE Midcap 250 index.

Among firms set to enter the FTSE 250 companies were conveyor-belt maker Fenner (FENR.L), student housing provider Unite Group (UTG.L), investment fund Genesis Emerging Markets (GENE.L) and cash-and-carry wholesaler Booker Group (BOK.L).

Meanwhile, Avis Europe (AVE.L), construction group Morgan Sindall (MGNS.L), drugmaker Vectura Group (VEC.L) and insurer Chaucer Holdings (CHU.L) will be demoted to the FTSE Small Cap index .FTSC

AGGREKO POWERS AHEAD

Shares in Aggreko have risen 78 percent so far this year, with the company valued at 2.18 billion pounds ($3.56 billion) at Wednesday's close.

The firm operates temporary power stations in countries such as Kenya and Uganda and rents out generators around the world to events including the Olympics and the Super Bowl.

In October this year, Aggreko's shares hit a record high after the company raised its full-year profit guidance due to the performance of its emerging markets business.[ID:nLF504013]

Meanwhile, Rentokil shares have had a poor run since entering the blue-chip index in September, falling 14 percent against a 4.5 percent rise in the FTSE 100 over the same period.

The support services firm, which issued a string of profit warnings in 2007 and 2008, has set out a five-year turnaround plan to improve operational efficiencies and costs, and to fix problem areas such as parcel delivery business City Link.

Rentokil shares recently came under pressure due to concerns among investors on the company's outlook for 2010.

Gold producer Petropavlovsk (POG.L), which had been the top contender in the race to enter the FTSE 100, missed the opportunity of promotion after its share price slid 3.2 percent this week.

On the other hand, tour operator Thomas Cook (TCG.L), which last week had looked the likeliest FTSE 100 demotion candidate is expected to have retained its blue-chip status after rallying around 3 percent in the past few days. ($1=.6121 Pound) (Editing by Simon Jessop)