"They decided together they want their big day in September, probably on one of St Tropez's beaches. Ashton is already going into space on Branson's Virgin Galactic flight and he has said Mila will definitely be at his side as his honeymoon present," a source said. 

Mila Kunis and Ashton Kutcher
Mila Kunis and Ashton Kutcher. Pic/ Santa Banta

The couple will join 600 space tourists, who are signed up for Virgin's SpaceShip Two flight which costs 124,000 pounds a seat. The craft is set to launch at Christmas after passing a test flight over California's Mojave Desert.