Former European Central Bank executive board member Juergen Stark has taken some parting shots at his old institution in a letter sent to the bank's 1,600 employees, German weekly Spiegel reports Sunday.

In a farewell letter to his former colleagues, he accuses the ECB's governing council of having made decisions "that have stretched the mandate of the ECB to an extreme." He reportedly sees risks that the central bank is increasingly operating "under fiscal dominance" because of its purchases of government bonds.

Furthermore, it is "an illusion to believe that monetary policy can solve the large structural and fiscal problems in the euro zone," he reportedly wrote. Stark allegedly went on to say that whenever in history a central bank has subordinated itself to budget policy, it had needed to make concessions with respect to its real task of maintaining the value of the currency.

Stark announced he was leaving the ECB board in September 2011, citing "personal reasons." But people familiar with the matter told The Wall Street Journal that Stark had resigned because of frustration over the bank's expanding role in backing the finances of troubled euro-zone countries. He officially left the ECB at the end of last year.

The ECB had no comment on the Spiegel story.