The 32-year-old opted for a bold sequin-striped dress, which she teamed with some quirky geometric earrings and a dazzling statement ring, completing the striking ensemble with a chic plaited up-do.
Sienna Miller dazzled in a metallic dress at the BFI London Film Festival
This was the British beauty's first public appearance since ex-fiancé Jude Law announced earlier that day that he and former girlfriend Catherine Harding were expecting a child together, however Sienna was all smiles as she joined co-star Steve Carrell at the glitzy evening.
"I can confirm that Jude Law and Catherine Harding are expecting a child together in the spring," read the statement released by Jude's representative.
"Whilst they are no longer in a relationship, they are both wholeheartedly committed to raising their child. They consider this a private matter and other than this confirmation no statement will be made. I ask that you respect the privacy of all parties involved and their families."
Sienna Miller and Jude Law got engaged on Christmas Day in 2004
Sienna and Jude began dating in 2003, and were engaged on Christmas Day in 2004, however the couple chose to separate in 2006 following the actor's revelation that he had been unfaithful.
While they rekindled their romance in 2009, after two years together they decided to end their relationship.
Since then, the striking actress has been dating fellow actor Tom Sturridge, who she is engaged to. In 2012, Sienna gave birth to their young daughter, Marlowe.