Sandra Bullock is being lined up to star in 'Gravity'.

The Oscar-winning actress is a strong possibility for the lead role in Alfonso Cuaron's sci-fi drama if Natalie Portman turns down the offer she was given for the part last week.

Despite Portman being first choice with Cuaron, sources claim Sandra is popular with Warner Bros. executives as they feel the expensive effects will need a big name to justify the money spent, Cinema Blend reports.

Portman was offered the role after Angelina Jolie pulled out and it had been reported she would sign a deal within the next week as filming needs to begin by the end of January due to the film's other lead star Robert Downey Jr. working on 'The Avengers' and 'Sherlock Holmes 2'.

'Gravity' tells the tale of a female astronaut who must find her way back to Earth after her space station is hit by satellite debris.

The female lead takes the majority of the screen time of the film - similar to Tom Hanks in 'Castaway' - leading to Warner Bros. wanting to get the casting for the $80 million project exactly right.