Media reports say the delay was due to concerns about evidence the prosecution refused to disclose to defense lawyers.
The court has not set a new start date for the trial.
Lubanga led the Patriotic Forces for the Liberation of Congo (FPLC), in the eastern part of the Democratic Republic of Congo during the country's 1998 to 2003 civil war. He has denied charges that he recruited thousands of children to fight in the conflict.
The International Criminal Court in The Hague was set up in 2002 to try the world's most serious cases of genocide, war crimes and crimes against humanity.
The case against Lubanga is the first to reach the trial stage at the war crimes court.