The 22-year-old actress has been cast as the young Lily is the as-yet-untitled series, which is set in 1980s Los Angeles.

It will follow Lily as she moves from New York to live with her estranged sister, wild-child Carol (played by Krysten Ritter, whose credits include Veronica Mars and Breaking Bad).

"They just cast Brittany Snow," Ritter told E! Online, confirming the news. "It's definitely official... It's going to be so good. I'm excited. I think that her and I together will have a great time, and going to centre around both of us and I think it's going to be good."

Snow and Ritter will first appear in an episode of Gossip Girl set to air in the US in May, which will introduce their characters via an '80s flashback.

If this backdoor pilot proves successful, the fully-fledged spin-off - tentatively titled Lily - is expected to premiere in the second half of 2009.

Snow previously starred in the TV series American Dreams and Nip/Tuck.