But at a press conference earlier on Monday she said: "I wouldn't say I was in an intense relationship with anyone at the moment."
And the N'Sync star won't be with the singer for the glittering gala, unlike the US première in February.
Meanwhile thousands of fans have gathered outside the Odeon Leicester Square for a glimpse of their idol.
Crossroads sees Spears play Lucy, who goes on a trip across the US with two friends to sort out problems in their lives.
It opens in the UK on Friday.