The agency, which is primarily focused on womenswear but also sells Givenchy menswear, will handle UK and international sales from autumn 08.
It has been tasked with expanding Powell's distribution to high-profit department stores in the US and Japanese markets.
Powell said: "We've probably got the coverage we want in the UK, and taking on an agency will give us time to concentrate on other parts of the business.
"Personally, I want to spend more time on the creative side. We've developed the range a lot this season, which is our fourth. Although we want to keep what's already there we need to keep developing the look."
Powell has confirmed that his Autograph menswear range for Marks and Spencer will run for a second season in autumn 08 after a successful first run in autumn 07.