Sienna Miller is engaged.

The Factory Girl actress and partner Oli Green welcomed their second child into the world earlier this year, and now it has been revealed they are taking another step in their relationship and getting married.

The 44-year-old star - who also has daughter Marlowe, 13, with former partner Tom Sturridge, and a two-year-old daughter with Oli - stepped out in Spain last week sporting a huge diamond ring on her engagement finger, walking with her hands in front of her so her jewellery could be seen, and E! News have confirmed the piece is an engagement ring.

The couple are largely private about their lives together, keeping their kids out of the public eye, and it only emerged Sienna had given birth to her third child when she let the news slip last month while promoting her latest project, Jack Ryan: Ghost War.

She told E! News: "It's happened. I have a tiny baby next door.

"It feels like stringing sentences together is a bit challenging.

"I'm on very little sleep but I'm madly in love with my baby."

The American Sniper star recetly admitted adjusting to her expanded family hasn't been easy.

Speaking on The Tonight Show last month, she said: "I would have said the teenage girl would be harder to juggle until the transatlantic flight I took yesterday with the toddler and the newborn.

"The toddler now wins hands down. It was an absolute disaster. There's no negotiating.

"The looks people give you on the flight. The baby's screaming, she's screaming. It was like, 'Yeah.' I got to the immigration line and just broke down."

Sienna - who has been dating Oli for four years - recently admitted she has found being pregnant in her 40s "so much easier" than her 20 because her life is in a "more grounded space" now.

She told Glamour magazine: "Having had a baby at 29, and then having a baby at 42, and now 44, it's so much easier when you don't have the conflict of feeling scattered and like you want to be doing X, Y, Z.

"If I'm in bed at 9 p.m. with a book, I'm so happy now.

"And now I've got the excuse to do it.

"Life is in a more grounded space. I think the 30s are chaos.

"You're like, 'I want to settle down. I want kids.' "

Sienna has also found being pregnant in her 40s easier because she doesn't care "what anyone else thinks".

She added: "But by the time you hit 40, you're like, 'I kind of know who I am. I don't really give a s*** about what anyone else thinks.'

"I'm a much more grounded human.

"We don't judge men who are having kids in their 80s. Why on earth is there any sort of narrative?"