He will now be leaving the Department of Justice at the end of September, but has not yet said what he will do next.

Joel Klein has been a champion for America's consumers

Attorney General Janet Reno

Probably his biggest success was a verdict against Microsoft, which found the software giant guilty of abusing its monopoly position in providing operating systems for personal computers. The software giant was ordered to split in two.

"I have done what I set out to do here, and our work is on the right track," Mr Klein said in a statement announcing his departure.

"Joel Klein has been a champion for America's consumers," Attorney General Janet Reno said. "He has fought tirelessly for marketplace competition, and Americans have enjoyed better products, more choices and lower prices as a result."

Mr Klein's time in the job was also marked by an increase in criminal prosecutions of international price-fixing cartels.

Microsoft in limbo

Microsoft is currently appealing the court decision that ordered it to split in two.

Mr Klein's resignation may yet slow the appeal process.

The US Supreme Court is set to announce on 2 October whether it will hear the appeal.

Microsoft has urged the Supreme Court to allow its appeal for anti-competitive behaviour to be heard by a lower court of appeal first. This court has found in Microsoft's favour previously.

The US Justice Department is hoping for a "fast-track" appeal.

Mr Klein's interim successor will be Douglas Melamed, now the principal deputy assistant attorney general in the antitrust division.

Until now Douglas Melamed was the No. 2 man in the division.

Mr Melamed is 55 years old and a graduate of Yale and Harvard Law School.