BERLIN (BNO NEWS) -- European and American stock markets plunged on Friday afternoon after European Central Bank (ECB) chief economist Jürgen Stark unexpectedly resigned 'for personal reasons', although analysts believe there is another reason.

A brief statement released by the ECB press office said Stark, who is a member of the Executive Board and Governing Council of the ECB, had informed President Jean-Claude Trichet that he will resign prior to the end of his term, which would have been May 31, 2014.

"Mr. Stark will stay on in his current position until a successor is appointed, which, according to the appointment procedure, will be by the end of this year," the statement said. "Having been informed by Jürgen Stark of his decision to resign for personal reasons, President Jean-Claude Trichet thanks him wholeheartedly for his outstanding contribution to European unity over many years."

The dramatic announcement came just before European stock markets closed and just before American stock markets opened. Frankfurt's DAX stock index plunged about 4 percent, while the Dow Jones Industrial Average dropped 2.69 percent to close below 11,000.

However, while the ECB only said Stark resigned for personal reasons, experts believe the chief economist resigned over a recent controversial decision by the ECB to buy bonds of struggling member states such as Greece. Stark was, along with other German officials, opposed to this decision.

Stark's decision to quit increased fears that Greece could default as soon as this weekend.