LONDON (Reuters) - Online food-delivery service Just Eat (>> Just Eat) will join Britain's leading share index in December, after rapid growth more than trebled its value since it was listed in 2014, FTSE Russell confirmed on Wednesday.

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

Its expansion won Just Eat (>> Just Eat) a promotion to the blue- chip FTSE 100 in the index provider's quarterly review of constituents, to take effect next month.

Currently trading on the mid-cap FTSE 250 index <.FTMC>, Just Eat shares have gained 20 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 798.5 pence on Wednesday at the close.

German rival Delivery Hero (>> Delivery Hero AG), which listed in late June, has soared 40 percent since then and was added to the pan-European STOXX 600 on Wednesday in another sign takeaway food apps are booming.

By contrast, British restaurant and pub chain Restaurant Group (>> The Restaurant Group) was ejected from the mid-cap FTSE 250 after years of slowing sales in a tough market for eating out.

Other additions to the top British share index were packaging group DS Smith (>> DS Smith) and safety device maker Halma (>> Halma). Oilfield services group John Wood (>> John Wood Group), which had been expected to join, was left out.

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

Dropping out of the large-cap index after their market capitalisation fell are medical technology company Convatec (>> ConvaTec Group), Madame Tussauds owner Merlin Entertainments (>> Merlin Entertainments) and defence and engineering contractor Babcock (>> Babcock International Group).

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

The new entrants to the FTSE 100 were 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 entering the mid-cap index will be newly-listed Austria-based construction materials firm RHI Magnesita (>> RHI Magnesita NV), used vehicle retailer BCA Marketplace (>> BCA Marketplace PLC), sweetener maker Purecircle (>> PureCircle Limited), engineering fluids maker TI Fluid Systems (>> TI Fluid Systems PLC) and investment fund F&C Global Smaller Companies (>> F&C Global Smaller Companies PLC).

Apart from Restaurant Group, stocks to be ejected from the FTSE 250 are P2P Global Investments (>> P2P Global Investments PLC), Vectura Group (>> Vectura Group PLC), Electra Private Equity (>> Electra Private Equity Plc), and PayPoint (>> Paypoint plc).

(Reporting by Helen Reid, editing by Keith Weir, Larry King)

By Helen Reid