Sienna Miller's childhood sweetheart used to be Jude Law look-a-like model Calum Best.

Sienna, 16 at the time, and Calum, then 17, were completely smitten by each other, a friend was quoted by Internet portal Femalefirst as saying.

"Sienna was really smitten with him and Calum was infatuated with her from the beginning. The young lovebirds would write to each other when Calum went back to Los Angeles. Eventually, though, the relationship fizzled out," the friend revealed.

Their longstanding friendship continued when Sienna housed Calum after he moved back to London five years ago. "Calum is really grateful for all the help Sienna gave him at that time he wanted to stay in the capital, but not with his dad. The pair had a lot of fun as flat-mates for three months," a source was quoted as saying.