The designer - who joined YSL as design director in 2000 after stints at Giorgio Armani, Prada and Miu Miu, and took over from the then-creative director Tom Ford in 2004 - has decided to part ways with the French fashion house although he has not announced where he is going.
Tom Ford and Stella McCartney have signed up to Livia Firth's Green Carpet Challenge.
The designers will join a host of other fashion houses including Chanel, Gucci and Yves Saint Laurent to create original eco-friendly dresses for Colin Firth's wife to wear on the red carpet during awards season.
Rachel Zoe has lost a suitcase full of designer clothes.
The celebrity stylist - who launched her own label earlier this year - saw her trunk full of treasured vintage clothing lost while she flew to the luxurious holiday destination of St. Barts with American Airlines.