It's reported the singer was lonely without Kevin Federline, and flew him from Los Angeles to see her perform in Wembley.
The Sunsays Spears has been "stressed out" by negative publicity surrounding her shows.
Federline is said to have joined Spears at the five-star Berkeley Hotel, in Knightsbridge, West London, and then watched the gig from beside the stage.
A source close to the singer said, "She was desperate to see Kevin. She has totally fallen for him and her aides are concerned that she is very distracted from her work as a result of the relationship.
"She is besotted with Kevin and it seems she can't function without him."
The pair's romance is said to have caused outrage in the US, where Federline's girlfriend - actress Shar Jackson - is seven months pregnant with their second child.
Speaking about her boyfriend's relationship with Spears, Jackson said, "This is a big thing. It isn't like breaking up a relationship, it's like breaking up a family. He can come home and we'll talk."
