LONDON, Aug 30 (Reuters) - Marks & Spencer will rejoin Britain's FTSE 100 stock index after a four-year hiatus, it was confirmed on Wednesday, capping a return to fashion for the food and clothing retailer after years of failed turnaround efforts.

Before its 2019 ejection, the food, clothing and homewares retailer, known affectionately as "Marks and Sparks", had been a member of the blue-chip index since its launch in 1984.

The relegation of the 139-year-old chain was a symbol of its painful decline, with investors losing faith after a decade of failed reinventions and three years of profit falls.

But a series of positive trading updates and a pledge to restore dividends has sent its shares up 80% this year, giving it a market value of 4.4 billion pounds ($5.6 billion).

The biggest turnaround has been in womenswear, where dresses such as a 39.50 pound floral midi smock have won over shoppers, sending casual dress sales up 40% and enabling it to better compete with the likes of Zara and Next.

Reducing the size of its range and sourcing more from locations such as Turkey, rather than Asia, has helped M&S cut the time it takes to order new items to around six weeks, enabling it to boost the availability of popular designs.

Shares in M&S, which reached a 19-month high of 235 pence earlier this month, closed on Wednesday at 224 pence.

"The market actually is still being quite churlish with the rating, and that probably is the once bitten twice shy thing," said Ian Lance, fund manager at Redwheel, M&S's biggest shareholder with about 8%.

"We genuinely think it is the real deal this time," he said, noting the potential for M&S to buy back shares.

Lance said plugging in M&S's profit margin targets of about 4% in food and over 10% in clothing and home - up from 3.4% and 8.7% last year - could deliver a share price north of 300 pence.

In its 2022/23 year, M&S made a pretax profit of 482 million pounds - less than half the 1 billion pounds-plus in 2008 - but analysts see it delivering more in the next three years, according to Refinitiv data.

The company upgraded its full-year profit outlook earlier this month, highlighting market share gains.

The return to the FTSE 100 is a coup for CEO Stuart Machin, who was promoted in May last year.

Under Machin, co-CEO Katie Bickerstaffe and chairman Archie Norman, M&S has also increased investment in technology and e-commerce, and overhauled its store estate.

The company's upmarket food business is shining too, after it increased ranges and lowered prices on staples.

It is the third fastest growing food seller in the UK after discounters Aldi and Lidl, according to market researcher NIQ. It also benefits from having older, more affluent customers during a cost-of-living crisis.

Its joint venture with online supermarket Ocado has not delivered as well as expected however, and Norman told shareholders in July he was "not happy with where it is".

A reset of the business is under way, aimed at improving the customer experience and reducing costs.

FTSE Russell, the index provider, confirmed M&S's return after the end of trading on Wednesday, with the change taking effect on Sept. 18.

($1 = 0.7913 pounds) (Reporting by James Davey Editing by Mark Potter, Kirsten Donovan)