The judge overseeing the perjury trial in Washington of former star major league baseball pitcher Roger Clemens has declared a mistrial over inadmissible evidence shown to the jurors.

U.S. District Judge Reggie Walton said Thursday that Clemens could not be assured a fair trial after prosecutors showed jurors evidence against his orders during the second day of testimony.

They played a video from Clemens' 2008 appearance before Congress during which he denied the use of banned, performance-enhancing drugs.

The judge said the portion that showed Clemens' close friend and former New York Yankees' teammate Andy Pettitte introduced evidence that appeared to bolster the credibility of a future witness. In expressing his anger, Judge Walton noted the case has already cost a lot of taxpayer money and that “government counsel should have been more cautious.''

Walton has scheduled a hearing September 2 to determine whether to hold a new trial.

The mistrial is a major setback for the government, which spent a year preparing for the case. Four days were spent on jury selection.

The judge said the two sides would have to discuss whether retrying Clemens would violate the constitutional protection against double jeopardy, which prevents an individual from being tried twice for the same offenses.

The 48-year-old Clemens pitched for the New York Yankees, Boston Red Sox, Toronto Blue Jays and Houston Astros during a stellar 24-year career. He was a seven-time winner of the Cy Young award, which annually honors the best pitcher in each league.

Clemens is one of only four pitchers to strike out more than 4,000 batters.