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She has been dating director Bennett Miller for more than a year, after they met on the set of the 2014 film Foxcatcher.
And Sienna Miller sparked rumours of a proposal on Wednesday night, after flashing a sizeable diamond ring on her engagement finger while stepping out in London.
Actress - Tom - Sturridge - Co-ords - Performance
The 35-year-old actress, who was previously engaged to actor Tom Sturridge, looked chic in flame-print co-ords as she left a performance of Cat On A Hot Tin Roof.
The blonde beauty looked typically chic in the silky printed pyjama shirt and loose matching palazzo pants.
Comfort - Alfie - Star - Pair - Flatform
Opting for comfort, the Alfie star completed her ensemble with a pair of white flatform trainers and accessorised with a black and white woven leather tote.
Smiling coyly, the British beauty seemed oblivious to the attention her dazzling ring commanded as she stepped out without her 50-year-old beau Bennett.
Outing - Piece - Jewellery - Sienna - Ring
It seems to be the first public outing for the significant piece of jewellery, as Sienna wasn't spotted wearing the ring at a fashion party last week.
MailOnline has contacted a representative for Sienna for comment.
Actress - Bennett - Set - Foxcatcher - Sources
The actress met Bennett on the set of Foxcatcher, with sources confirming to MailOnline in June that they are 'happily' still together.
The pair keep their romance under wraps, and were last seen in public at the US Open last September.
Sienna - Bennett - Share - Surname - December
Sienna and Bennett, who share the same surname and whose December birthdays are just two days apart, were alsp seen attending Wimbeldon twice together in July that year.
They had reportedly been dating in secret for...
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