Rumour has it that British actor Robert Pattinson has given his girlfriend Kristen Stewart an ultimatum in their relationship.

Twilight couple Robert Pattinson and Kristen Stewart may not be back together for good. Rob is said to have given Kristen her last warning that they will break up if she will not change her ways.

Kristen and Rob seemed to have rekindled their relationship as they appeared very close at the premieres and promo tour of 'Twilight: Breaking Dawn - Part Two', as well as being spotted sharing flights together. However, according to Now magazine, getting back together was on a "trial basis".

The couple's placid relationship was disrupted by Kristen's cheating scandal with her "Snow White and the Hunstman" director Rupert Sanders. Apparently, Rob will decide their future by Valentine's Day (February 14) next year.

"Rob's planning to skip most of the upcoming awards parties and spend January focusing on his relationship. He's set a Valentine's Day deadline to decide whether he's in it for the long run. So there could be a proposal on the cards - or a decision to end it," a source told Now magazine.

Rob is set to fly to Australia next year to film his new movie "The Rover". While Kristen might have opened a wound with her boyfriend Rob, as she recently confirmed that she will be starring in a "Snow White And The Huntsman" sequel.

Reports said that Rupert Sanders was booted out of the film and will not be involved in the new movie. However, the new director is yet to be confirmed.

Other reports also said that the Twilight couple will star in the remake of Elizabeth Taylor and Richard Burton 1965 film "The Sandpiper" - a film about a school teacher who has an affair with a single mother.

Some close to the British actor said, "He [Rob] recognizes there's a certain screen magic when he and Kristen are together. Besides, there's less chance she'll get in trouble again."

Now, Rob and Kristen are currently spotted in London and celebrating the New Year together.