The British pair - who started a romance on the set of Alfie in 2004 but split two years later after Jude had an affair with his children’s nanny - have had a series of secret meetings in New York where they are both currently starring in Broadway shows.
A source said: Sienna and Jude have met up a few times and got on really well.
Sienna had her heart broken by Jude, but she was young and it was the first time she’d fallen in love.
What people don’t realise is that they always remained close, so who knows where this will lead to next.
Jude recently hit the headlines after fathering a daughter, new born Sophia, following a brief fling with aspiring model Samantha Burke, and insiders claim they want to keep their romance under wraps to avoid further controversy.
The source added: They're desperate to keep their hook-ups secret because their break-up was so public. They don't want any potential reconciliation all over the papers, especially with Jude having just fathered a child with another woman.
In order to keep their secret, the couple have enlisted the help of Johnny Lee Miller, who is currently starring in After Miss Julie with Sienna.
The source explained to Now magazine: Jonny's great at throwing the paparazzi off the scent. He'll walk down the street with Sienna to where Jude's staying just round the corner.
A spokesperson for Jude - who is currently starring in play Hamlet -has confirmed the former couple have met up, while the Factory Girl actress who still wears the diamond engagement ring given to her by the actor has insisted there is nothing weird about them spending time together.
She said: "People would love to make a whole meal out of it, but I've seen 'Hamlet' twice now and there's no weirdness for me personally that he's there and I'm here."
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