Jessica Simpson's boyfriend has been accused of cheating on her.

American football player Tony Romo reportedly visited Hollywood hotspot My House on 16 January — hours after the 28-year-old singer had flown out of Los Angeles — and then invited a group of people back to the home he shares with her.

With the party in full swing, it is alleged Romo — who has been dating Simpson since November 2007 — disappeared upstairs with a stunning brunette for the rest of the evening.

A source told America’s Star magazine: "Everyone was talking about it downstairs. The girls were saying it was so wrong and shady of Tony to cheat on Jessica — especially in her place! I just can't believe he would hook up with another woman at his own girlfriend's house."

Neither Simpson or Romo have commented on the claims.

Jessica has recently been the subject of scathing comments about her seemingly fuller figure, prompting her sister Ashlee to leap to her defence.

Ashlee wrote on her blog: "I am completely disgusted. Since when did a woman's weight become newsworthy? How can we expect teenage girls to love and respect themselves in an environment where we criticise a size two figure?

"I find it completely embarrassing and belittling to all women to read about a woman's weight or figure."