Spielberg - whose latest film is the futuristic Minority Report - told Empire magazine that Lucas was too possessive of his blockbuster space saga to let anyone else have a try.
Lucas and Spielberg have worked together several times including on the Indiana Jones series of movies.
But, despite their friendship and Spielberg's Oscar-winning credentials, Lucas considers the Star Wars series his "baby".
"I've asked him. He won't let me do one," said Spielberg in the interview.
"I wanted to do one 15 years ago and he didn't want me to do it. I understand why - Star Wars is George's baby. "
Lucas's latest Star Wars instalment took box offices by storm around the world when it was released in May this year.
Star Wars: Episode II - Attack of the Clones, the fifth instalment in the cult Star Wars series that began in 1977, was one of the most hotly-anticipated films of the year.
It opened to warmer reviews than Episode I - The Phantom Menace, which was widely seen as a disappointment when it was released in 1999.
Popular classics
Spielberg too is no stranger to producing box office hits.
His back catalogue includes some of the most popular movies of recent cinemas history, such as E.T and Jurassic Park.
Minority Report - released in the UK and US on 4 July - has also been well received.
It stars Tom Cruise in a bleak thriller set in 2054.
And Spielberg is soon to begin the fourth Indiana Jones adventure - again starring Harrison Ford - after a gap of more than a decade.
"The fun of it is I get to revisit a bit of my childhood again," Spielberg explained of his decision to do another Indiana Jones film.
"It's a chance for all of us to go back and feel young and act young. Harrison is going to be full of energy, as he always is."
He concluded: "It's going to be very exciting. Even though he will be 62 by the time this film comes out, Harrison hasn't lost the snap in his whip."
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