Sienna Miller ''can't wait'' to marry Lucas Zwirner.

The '21 Bridges' actress was believed to have gotten engaged to the publisher earlier this year after their romance began in December 2018, and sources have now said she's already planning her nuptials and is ''looking forward'' to tying the knot.

An insider told Us Weekly magazine: ''[Sienna] can't wait to make him her husband. They are both looking forward [to the wedding].

''[Lucas] loves how family-oriented Sienna is, and she loves that Lucas is youthful and fun.''

Sienna - who has seven-year-old daughter Marlowe with her ex-partner Tom Sturridge - and Lucas, 29, were first rumoured to be engaged in February, but as of the time of writing, the couple are yet to confirm speculation.

A source said at the time: ''They're so in love.

''Her daughter has a nice and close relationship with Lucas too. They're excited for this new chapter.''

The 38-year-old actress had also sparked engagement speculation the month before when she was spotted wearing a diamond on her ring finger in New York City.

In photographs obtained by MailOnline, Sienna proudly flaunted her jewellery as she held onto her coat with her left hand.

Their engagement didn't come as too much of a surprise to their friends as Sienna had already said she ''loves'' Lucas and she would be open to marrying him in the near future.

She gushed: ''It is love.

''It's nice and settled and happy. Yes, I would love to have more children. I would love a daughter as well. I think I would be open to getting married, yes.''

The 'Alfie' star went public with her new relationship last year after she and art gallery owner were introduced to one another through mutual friends.

She said: ''He's not an actor! He's not famous!''

Meanwhile, Sienna previously said she wished she had her daughter Marlowe younger because she would like to have grandchildren ''now.''

She said: ''I wish I'd had my daughter younger, and I could have grandchildren now. I'd like more children at my age now, too.''