Robert Pattinson Gives Kristen Stewart Ultimatum Over Boyfriend?

Robert Pattinson has reportedly issued Kristen Stewart with ultimatum after apparently growing tired of waiting for her to chose between him and her long-term boyfriend.

Insiders say things are hotting up between the Twilight saga co-stars – dubbed Robsten – but Kristen is still involved with actor Michael Angarano and has so far shied away from committing to Rob.

"Robert is completely frustrated," a source tells OK!. "If Kristen isn't interested in a serious relationship, she needs to tell him so he can move on."

Rob - who is currently preparing to start shooting new movie Remember Me in New York - has given Kristen until their reunion on the Vancouver set of Eclipse to make up her mind once and for all, it's claimed.

"He's asked her to choose between him and Michael this summer," says the source. "Robert's not willing to be strung along."

Rob and Kristen at the MTV movie awards