General David Petraeus has resigned as Chief of the Central Intelligence Agency because of an extramarital affair.

In a letter to President Barack Obama, General Petraeus wrote that he used extremenely poor judgement and that such behavior is unacceptable in a leader.

The president accepted the resignation and is expected to make a formal statement later Friday.

Petraeus was a four-star general when he took over as American intelligence chief last year after a long career in the U.S. military.

He was the multi-national force commander in Iraq and took over as commander of U.S. forces in Afghanistan in 2010, becoming one of the world's best known military leaders in recent years.