European Central Bank President Mario Draghi may wait until Germany's Constitutional Court rules on the legality of Europe's permanent bailout fund before unveiling full details of his plan to buy government bonds, two central bank officials said.

With the court set to rule on Sept. 12, investors looking for Draghi to announce a definitive purchase program at his Sept. 6 press conference might be disappointed, according to the officials, who spoke on condition of anonymity because the deliberations are not public. The program is still being worked on and staff may not be able to finalize it by then, said the officials, who are familiar with thinking on the ECB Governing Council. An ECB spokesman in Frankfurt declined to comment.

Draghi announced on Aug. 2 that the ECB may intervene in the secondary market to reduce bond yields in countries such as Spain and Italy if they apply to Europe's bailout fund for aid and accept the conditions attached. The European Stability Mechanism, intended to replace the temporary European Financial Stability Facility, hasn't entered into force yet as legal wrangling over its compatibility with the German constitution continues.