The American judge in former baseball star Roger Clemens' perjury trial has declared a mistrial after ruling that prosecutors violated his orders regarding the handling of evidence.

US District Judge Reggie Walton said he made the decision because Clemens could not be assured a fair trial when prosecutors went against his orders and showed inadmissible evidence to jurors on just the second day of testimony.

Walton then scheduled a September 2 hearing to decide whether to have a new trial.

"There are rules that we play by and those rules are designed to make sure both sides receive a fair trial," Walton said.

"The ability with Mr. Clemens with this jury to get a fair trial with this jury would be very difficult if not impossible."

Prosecutors then asked the judge if the jury could simply be instructed to disregard what they had been told but Walton replied, "I don't see how I un-ring the bell."

The drama began when the judge interrupted the prosecution's playing of a video of Clemens' testimony before the US Congress three years ago and then asked the jury to leave the courtroom.

Clemens is accused of lying during that testimony when he insisted he never used performance-enhancing drugs during his Major League Baseball career.

Clemens, 48, was named in the Mitchell Report in December 2007 as one of 86 players who took performance-enhancing drugs but he steadfastly denied the allegations and pushed for a hearing before Congress.

Clemens, nicknamed "Rocket", is one of only four pitchers to have have made more than 4,000 strike-outs during his career.

He won the World Series twice with the New York Yankees in 1999 and 2000. He also played for Boston Red Socks, Toronto Blue Jaysand Houston Astros during his 23-year career.