Japan's Olympics minister says organisers have not worked out where to place the ceremonial cauldron in Tokyo's new flagship stadium.
The wooden interior of the new design means installing an internal cauldron could violate Japanese fire codes.
The stadium has been hit by delays and controversy over the cost of an earlier design by Zaha Hadid.
A cheaper design, by Japanese architect Kengo Kuma, was chosen in December last year.
The new design will cost 149bn yen ($1.2bn; £825m) to build.
Ms Hadid's plan would have cost 252bn yen, making it the world's most expensive sports venue.
The International Olympic Committee deadline for completion is January 2020, ahead of the Games' start in July that year.
"It appears things were proceeding under the old plans without much discussion [of where to put the cauldron,]" Olympics Minister Toshiaki Endo said.
"The lighting of the flame is the main event of the Olympics. As for exactly how it will be done and where the cauldron will be, that will be discussed at a later date," he added.
He said that he hoped a solution to the problem would be found before April.
Mr Endo will head a panel set up to review the situation of the cauldron.
The International Olympic Committee stipulates that the cauldron should be placed so that it is easy to see for spectators.
Designers had considered placing the cauldron outside the stadium, but this was met with opposition from Japan's Olympics committee.
The Olympic flame is lit in Olympia, Greece, before travelling across the world in a torch relay.
The lighting of the Olympic cauldron by the final torchbearer marks the start of the Games.