She joins previously announced castmembers Ben Whishaw and Matthew Goode in Julian Jarrold's forthcoming adaptation of the Evelyn Waugh classic, adapted by Andrew Davies and Jeremy Brock.
The project will shoot from late May 2007 on location in Oxford, Venice and Morocco.
Ecosse Films' Robert Bernstein and Douglas Rae are producing alongside Kevin Loader. HanWay Films is handling sales and will announce a North American deal soon.
Atwell, 25, was one of Screen International's 2006 Stars Of Tomorrow. She starred in Saul Dibb's BBC project The Line Of Beauty and was cast by Woody Allen for his next film Cassandra's Dream.
'Hayley Atwell's star is certainly in the ascendant,' said HanWay CEO Tim Haslam. 'She is a dynamic and talented young actress, who we believe is the perfect choice to portray Julia Flyte, the conflicted romantic heroine of Brideshead Revisited,' commented Tim Haslam, CEO of HanWay Films.
Bernstein added: 'Casting Hayley Atwell completes a trio of exciting, up and coming cutting edge British actors, which makes Brideshead a particularly thrilling prospect.'