TOKYO — Japan said Monday that radioactive emissions from the stricken Fukushima Daiichi nuclear power plant in the early days of the March 11 earthquake and tsunami disaster might have been more than twice as large as a previous estimate, suggesting the accident was more grave than the government had publicly acknowledged.

It is unclear whether a more accurate reading of emissions levels would have promoted a swifter or wider evacuation from around the plant. Still, the lag in reporting the true extent of the emissions added to what some critics have called a litany of confusing and contradictory data and analysis from the Japanese authorities, putting officials on the defensive about whether they delayed, or even blocked, the release of information to the public.

Last month the government acknowledged that three of the plant’s reactors had probably suffered fuel meltdowns, after having denied that possibility.

On Monday, Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency, said that the reactor pressure vessel at one of the plant’s reactors appeared to have been compromised as early as five hours after the quake.