Justin Timberlake at the Alpha Dog premiere

Rumours that Justin Timberlake and Cameron Diaz have split were given more credibility last night when the singer turned up to the premiere of his new movie alone.

Sources in America have said 25-year-old Justin split with 34-year-old actress Diaz shortly before Christmas. And last night Timberlake added to the speculation by attending his film Alpha Dog without Diaz.

Timberlake, who enjoyed a long term relationship with Britney Spears, looked lonely and shy at Hollywood's Arclight cinema.

Hardly surprising, as it is only his second movie, so he could have probably done with all of the help he could get from seasoned Cammie.

Cameron's filming schedule is clear, she has had a long break after The Holiday in England, and she doesn¡¯t start her next project, the third Shrek, until next month. So no professional reason they could not be together.

Our spy at the premiere of the movie, which stars Bruce Willis and tells the life story of a Hollywood drug dealer who was one of the youngest men ever to be on the FBI¡¯s wanted list, said: "Justin did crack a few smiles but generally looked a bit down."

Since meeting at the Nickelodeon Kids' Choice Awards in 2003 Justin and Cameron have defied the rule that A-list couples don't last long.

Their relationship has been the subject of many rumours - not the least of which being that the couple planned to marry.

However, despite their long term love, they have not hinted at getting married or even becoming engaged.