* Medical examiner to determine cause of death
* Boyfriend Mick Jagger 'struggling' to understand suicide (Adds tour cancelled, Jagger comment)
By Michael Roddy and Patricia Reaney
LONDON/NEW YORK, March 18 (Reuters) - The Rolling Stones called off their tour in Australia and New Zealand on Tuesday after lead singer Mick Jagger's girlfriend L'Wren Scott was found dead of an apparent suicide in her Manhattan apartment.
Scott, a 49-year-old fashion designer and former model, was found hanging from a scarf on Monday in a high-rise apartment building in the upscale Chelsea neighborhood.
Police are treating her death as a suicide and the medical examiner's office said tests to determine the exact cause of death were set for Tuesday.
"The Rolling Stones are deeply sorry and disappointed to announce the postponement of the rest of their 14 ON FIRE tour of Australia and New Zealand following the death of L'Wren Scott," the group said in a statement.
The group's Australian publicist asked fans to hold on to their tickets until an update was made. The group had been due to kick off a seven-concert Australia and New Zealand leg of their world tour on Wednesday in Perth in western Australia.
Jagger, 70, who had been dating Scott since 2001, said he had spent many wonderful years with the acclaimed designer.
"I am still struggling to understand how my lover and best friend could end her life in this tragic way," he said in a post on his official website, mickjagger.com. "She had great presence and her talent was much admired, not least by me."
Scott's publicist said funeral details had not been confirmed.
News of Scott's tragic death shocked friends, clients and fellow designers. It was unclear if the designer had shown any signs in her work or personal life indicating she was depressed or suicidal.
Although Hollywood stars such as Nicole Kidman, Amy Adams and Penelope Cruz were pictured in Scott's sleek, form-fitting creations on the red carpet, her company had mounting debts.
Accounts filed with Britain's Companies House in October show that Scott's LS Fashion Ltd ran a loss of 4.3 million euros ($5.99 million) in 2012, up from 3.0 million euros the year before.
Last month she cancelled her show at London Fashion Week, citing production problems.
Scott became one of New York's most famous designers over the last decade and was best known for her slim-fitting, dresses in beautiful fabrics and fine details.
Pop singer Madonna in a statement described Scott's death as "horrible and tragic." Supermodel Naomi Campbell, who knew Scott for many years, said she was the "epitome of elegance and femininity."
Scott and Jagger were often seen at celebrity events, with the 6-foot, 3-inch (1.91-metre) tall former model towering over her rock-star boyfriend.
A publicist for Jagger denied a New York Post story about a split between the two, saying it was "100 percent untrue."
Banana Republic, which had collaborated with Scott on a small holiday collection, said her death was terrible news.
"We are stunned and saddened to learn of the loss of L'Wren Scott. Our hearts and prayers go out to her friends and family," it said.
Scott, born Luann Bambrough, was raised in Roy, Utah with two older siblings by adoptive parents. Local residents were proud of her success and remembered her as very personable.
"She was a sweet young lady who had great dreams and lived to see those dreams fulfilled until now, I guess," said Richard Jensen, an 80-year-old retired electrical contractor who lived across the street from her family in Utah.
"It is quite shocking really. We felt she was so successful and had a great career," he added in a telephone interview.
Jensen watched Scott and her brother and sister grow up. Although she left Utah after her high school graduation, he said she often returned to visit her mother, who died about four years ago.
Scott started her career as a model in Paris before becoming a stylist and designer.
Her love of fashion began when she made her own clothes as a teenager, according to her website. After moving to Los Angeles, she worked as a stylist and designed privately before creating her own collection.
($1 = 0.7180 Euros) (Additional reporting by Victoria Cavaliere, Edith Honan, Piya Sinha Roy, and Belinda Goldsmith; Editing by Mary Milliken and Cynthia Osterman)