If the report determines that US and international law have been violated, the Biden administration would be expected to stop sending military assistance to Israel.
The administration emailed Congress notifying lawmakers that it will miss the deadline to submit the report but did not provide additional details.
When the National Security Council was asked to explain the delay, they referred any inquiries to the State Department.
Earlier on Tuesday, State Department spokesperson Matthew Miller said that the US government is trying very hard to meet the "self-imposed deadline."
Miller said it is possible to "slip" a little bit, but the administration is trying to get the report done by Wednesday.
On Monday night, some 200 attorneys, 27 of whom are currently in the Biden administration, sent a letter to top US officials arguing that sending weapons to Israel would be illegal.
The report's delay comes as Israel started a military operation in Rafah in the southern Gaza Strip, where some 1.4 million Palestinians - are sheltering.