Keira and Sienna best of friends on and off screen

The girls, who are currently filming The Edge Of Love - a film about Dylan Thomas - together, hit the town last night with Keira's fella Rupert Friend and their other co-star Matthew Rhys in tow.

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The girls linked arms and stayed in step during their night out in London

The girls are proving to be inseparable

Sienna's boyfriend Jamie Burke was suspiciously absent, despite the fact he is in London at the moment, but it clearly wasn't stopping Sienna from having a good time.

The gang went for dinner in Archipelago before having a very schoolkid moment of linking arms while running down the street.

In the film the two feisty, free-spirited women are connected by the charismatic poet, who loves them both.

It is understood that the script features a sexy threesome with the two women and the poet, and has been described as "very steamy" by those who have seen a sneak preview.

As we revealed earlier this week, 25-year-old Sienna stepped into the role of Caitlin MacNamara - Thomas's wife - in the movie when Lindsay Lohan pulled out after she was asked to take a drugs test.

We're told Sienna and 22- year-old Keira have become inseparable on the set in New Quay, Wales, so much so they are currently househunting for neighbouring cottages overlooking the sea there.

We're told: "Sienna and Keira feel it would be great to have homes there and go on joint holidays once filming stops, so they can keep up their friendship.

The source added: "They're actively hunting for the perfect cottages in the area."

Hello boys! Keira stepped out with her boyfriend Rupert Friend while Sienna teamed up with another pal

The film is set to feature a steamy threesome with Keira, Sienna and co-star Matthew Rhys

Sienna was cast after Lindsay Lohan pulled out of the movie

The actresses have been filming parts of the movie in Wales