Just Eat to join top table of British business

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* Strong performance delivers Just Eat to FTSE 100

* Other potential entrants are DS Smith, Halma and John Wood

* Demotions set to include Babcock, Mediclinic, Merlin

By Helen Reid

LONDON, Nov 27 (Reuters) - Online food delivery service Just Eat is set to join Britain's leading share index this Wednesday thanks to rapid growth that has seen its value more than treble since it was listed in 2014.

In a sign of shifting consumer tastes and investor appetite, Just Eat is now valued at around 5.5 billion pounds ($7.35 billion), more than high street stalwart Marks and Spencer and supermarket Sainsbury's.

Its expansion will win Just Eat promotion to the blue chip FTSE 100 in the index provider's quarterly review of constituents on Wednesday.

Currently trading on the mid-cap FTSE 250 index, Just Eat shares have gained 23 percent this quarter, boosted to a record high after strong results. Regulatory clearance of its takeover of Hungryhouse provided added extra impetus for a company that operates from Australia to the Americas.

Listed at a price of 260 pence in April 2014, Just Eat shares traded at 815 pence on Monday afternoon.

German rival Delivery Hero, which listed in late June, has soared 44 percent since then. A 60-percent jump in third-quarter revenue reported on Monday added to evidence takeaway food apps are booming.

By contrast, British restaurant and pub chain Restaurant Group looks set to be ejected from the mid-cap FTSE 250 after years of slowing sales in a tough market for eating out.

Other contenders to join the top British share index were packaging group DS Smith, safety device maker Halma and oilfield services group John Wood.

DS Smith was also trading at record levels, while Halma has been one of the best-performing mid-cap stocks, up 44 percent so far this year.

RELEGATION CANDIDATES

Likely to drop out of the large-cap index after their market capitalisation fell were South African private healthcare firm Mediclinic, medical technology company Convatec , Madame Tussauds owner Merlin Entertainments and defence and engineering contractor Babcock.

Babcock was on track for an 18 percent fall this month after an earnings update dented the shares, with the company and investors forecasting slower British defence spending.

Mediclinic shares, meanwhile, have sunk 19 percent so far this quarter to a four-year low, weighed by the collapse of plans to take over Spire.

The new entrants to the FTSE 100 will be announced this Wednesday, based on Tuesday's closing prices, with changes effective from the market close on Friday Dec 15, to start trading on Dec 18.

Small-cap stocks eligible for entry into the mid-cap index were newly-listed Austria-based construction materials firm RHI Magnesita, used vehicle retailer BCA Marketplace , sweetener maker Purecircle and engineering equipment provider Fenner.

Apart from Restaurant Group, stocks likely to be ejected from the FTSE 250 are P2P Global Investments, Vectura Group and Electra Private Equity. ($1 = 0.7484 pounds) (Reporting by Helen Reid; Editing by Keith Weir)