* Strong performance delivers Just Eat to FTSE 100
* Other additions are DS Smith and Halma
* Babcock, ConvaTec, Merlin demoted from index (Updates with confirmed index changes from FTSE Russell on Wednesday)
LONDON, Nov 27 (Reuters) - Online food-delivery service 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 ($7.35 billion), more than high street stalwart Marks and Spencer and supermarket Sainsbury's.
Its expansion won 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, 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, 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 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 and safety device maker Halma . Oilfield services group John Wood, 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.
Dropping out of the large-cap index after their market capitalisation fell are medical technology company Convatec , Madame Tussauds owner Merlin Entertainments and defense and engineering contractor Babcock.
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 defense 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, used vehicle retailer BCA Marketplace , sweetener maker Purecircle, engineering fluids maker TI Fluid Systems and investment fund F&C Global Smaller Companies.
Apart from Restaurant Group, stocks to be ejected from the FTSE 250 are P2P Global Investments, Vectura Group , Electra Private Equity, and PayPoint. ($1 = 0.7484 pounds) (Reporting by Helen Reid, editing by Keith Weir, Larry King)