The chief executive of German stock exchange operator Deutsche Boerse has resigned amid an investigation into potential insider trading.

The company's board announced Carsten Kengeter's departure on Thursday, just days after a German court turned down a settlement offer that would have closed the investigation.

Prosecutors in Frankfurt have been probing Kengeter's purchase of Deutsche Boerse (DBOEF) shares worth €4.5 million ($5.3 million) in December 2015, just weeks before his firm began merger talks with the London Stock Exchange (LNSTY).

Plans to combine the companies were announced in February 2016, and shares in both firms soared as a result. The deal was blocked by the European Union's competition watchdog in March.

Deutsche Boerse said earlier this year that Kengeter was allowed to buy the shares as part his official remuneration program, which was approved by its supervisory board.

The board, which had been publicly supportive of Kengeter, said Thursday that it had accepted his resignation with "deep regret." He will leave his post at the end of 2017.

Deutsche Boerse said in September that it had agreed a €10.5 million ($12.3 million) settlement with prosecutors that would have closed the investigation. The deal required Kengeter to personally pay an additional €500,000 ($585,000).

But the company said the judge overseeing the case rejected the settlement.

A representative for the prosecutor's office declined to comment on Thursday.