The QPR keeper made an appearance at Luiz's event wearing the home strip of the defender's team, and Cesar's club have intimated that the Brazilian may face disciplinary procedures for his actions.
A statement on QPR's official website read: "The club is aware of an image of Julio Cesar circulating on social networking feeds.
"The image, which appeared on an Instagram account, has since been deleted. The club will be dealing with this issue internally."
The former Inter Milan goalkeeper added to the release and claimed that he did not mean to antagonise anybody with his actions.
"I would like to clarify that last night I attended David Luiz's fancy dress birthday party and, as a joke for my friend, my family and I dressed as him," said Cesar.
"I want to assure everyone that absolutely no offence was intended."
Harry Redknapp's side play Reading at the weekend, with the losing team confined to relegation from the Premier League.