Speculation is growing that the Japanese government may start talks to nationalise Tokyo Electric Power, which owns the Fukushima nuclear plant.
Conflicting reports in local media caused nervousness among investors and Tepco's share price fell by 18%.
Tepco's future has been in question after the earthquake that struck Japan caused a radiation scare at Fukushima.
The company has said it will need to raise about $25bn (£15.6bn) to shore up its finances.
The talk of Tepco being nationalised has been fuelled by a statement from cabinet minister Koichiro Gemba to the Reuters news agency that a discussion about bailing out Tepco was possible.
But Chief Cabinet Secretary Yukio Edano said the government was not currently considering a nationalisation.
"Although details cannot be seen such as how exactly the government is going to nationalise the company, as long as there are concerns that Tepco may be nationalised, investors don't want to hold the stock," said Hajime Nakajima of Cosmo Securities.
On Tuesday a flood of sell orders caused Tepco shares to stop being traded temporarily.
A day earlier, the shares dropped to their lowest level in three decades.
A spokesman for Tepco told the Reuters news agency that he was unaware of any plan for nationalisation.
"Our first and biggest priority at this moment is to prevent the nuclear power plant accident from worsening further," Hajime Motojuku said.
However, The Yomiuri Daily has reported that some members of the government proposed a plan for the state to take a majority stake in the company and help it pay for damages from the nuclear accident.
Some Japanese banks have said they are in talks to give the company emergency loans.