Sienna Miller and her sister design for catwalk

Sienna Miller is to make her London Fashion Week debut next month after securing a prime-time slot to show off a collection she designed with her sister.

The news is a breakthrough for the pair, who launched the Twenty8Twelve by s miller label in August 2007.

Family flair: Sienna, far left, and Savannah will feature in London Fashion Week

The Hollywood star-turned-fashion designer has secured a prized evening slot at Fashion Week, which begins on 20 February.

This year's event, reduced to just five days to accommodate a change in the dates of New York Fashion Week, features up-and-coming stars including Central Saint Martins graduates Roksanda Ilincic and Christopher Kane alongside the likes of Vivienne Westwood and Paul Smith.

Miller, 26 , co- designs Twenty8Twelve with her 30-year-old sister Savannah. The name of the label refers to Sienna's birthday, 28 December.

Although the Bafta-nominated actress has no formal fashion qualifications, she is regularly named as one of the world's best-dressed women and her boho chic style sparked an obsession with gipsy skirts and cheesecloth blouses.

Savannah Miller studied at Central Saint Martins, graduating in 2004 with first-class honours.

She worked for Alexande r McQueen and Matthew Williamson before teaming up with her sister.

The Millers opened their first shop in Westbourne Grove in August 2007, and recently opened a second store in the designer fashion section of the new Westfield centre in Shepherd's Bush. Their collection is also stocked at Harvey Nichols, Liberty and Paris and Los Angeles boutiques.