In the deal, BP Products agreed to plead guilty to violations of federal law related to the blast and pay a fine of $50 million (26 million pounds).
The lead attorney for blast victims opposing the plea agreement said his clients were shocked the appeals court did not cancel the plea deal outright.
"The Fifth Circuit has sent this case back to the district court to carefully consider the victims' objections," David Perry said in a statement. "We are confident that when those objections are considered, this sweetheart plea bargain will be ultimately rejected."
Another attorney representing victims called Wednesday's ruling "a complicated decision" because it leaves the final resolution in the hands of U.S. District Court Judge Lee Rosenthal.
"I think the victims have a very strong argument that this was not a good plea agreement," said Paul Cassell, a University of Utah law professor working on behalf of the victims.
Among the failings, victims have said, is the small size of the fine compared with BP's profits, which reached a record $7.62 billion in the first quarter.
The proposed deal also lacks a long-term safety monitor at the refinery, where three workers have died in accidents since the 2005 explosion, Cassell said.
The Fifth Circuit's opinion praised Rosenthal's efforts to include the victims when she reviewed the plea agreement, which was negotiated under a different judge's supervision.
Fifteen workers were killed and 180 others were injured in the March 23, 2005, explosion at BP's refinery in Texas City, Texas, about 30 miles (48 km) south of Houston.
BP has accepted responsibility for the explosion, which a U.S. Chemical Safety Board investigation found was due in part to cost-cutting and poor maintenance at the refinery.
BP has paid more than $1.6 billion in legal claims from the 2005 explosion and a fine of $21.4 million to the U.S. Occupational Safety and Health Administration.
