Robert Pattinson has fallen for Twilight co-star Kristen Stewart in a very big way but the feeling is not entirely mutual, according to a new report.

Rumours that the pair are an item – or at least regularly 'hooking up' - have been gathered pace in recent weeks, and now insiders say things are getting serious - at least for Rob.

"Rob’s dedication to winning Kristen’s heart says something," a source tells Britain's OK! magazine "He’s falling for her fast and seems to be head over heels.”

Kristen may be less keen on taking their relationship to the next level.

Another source says: “Kristen told Robert she’s incredibly fond of him, but doesn’t want to race into a serious relationship.

“And she made it clear his constant pursuit is putting too much pressure on her.”

This follows a US report that the pair were all set to move in together.

Rob and Kristen at the Teen Choice Awards