Mycle Schneider is an independent consultant who has previously advised the French and German governments.
He says water is leaking out all over the site and there are no accurate figures for radiation levels.
Meanwhile the chairman of Japan’s nuclear authority said that he feared there would be further leaks.
The ongoing problems at the Fukushima plant increased in recent days when the Tokyo Electric Power Company (Tepco) admitted that around 300 tonnes of highly radioactive water had leaked from a storage tank on the site.
Moment of crisis
The Japanese nuclear energy watchdog raised the incident level from one to three on the international scale that measures the severity of atomic accidents.
This was an acknowledgement that the power station was in its greatest crisis since the reactors melted down after the tsunami in 2011.
But some nuclear experts are concerned that the problem is a good deal worse than either Tepco or the Japanese government are willing to admit.
They are worried about the enormous quantities of water, used to cool the reactor cores, which are now being stored on site.
Some 1,000 tanks have been built to hold the water. But these are believed to be at around 85% of their capacity and every day an extra 400 tonnes of water are being added.
“The quantities of water they are dealing with are absolutely gigantic,” said Mycle Schneider, who has consulted widely for a variety of organisations and countries on nuclear issues.
“It is much worse than we have been led to believe, much worse,” said Mr Schneider, who is lead author for the World Nuclear Industry status reports.
At news conference, the head of Japan’s nuclear regulation authority Shunichi Tanaka appeared to give credence to Mr Schneider’s concerns, saying that he feared there would be further leaks.
“We should assume that what has happened once could happen again, and prepare for more. We are in a situation where there is no time to waste,” he told reporters.
The lack of clarity about the water situation and the continued attempts by Tepco to deny that water was leaking into the sea has irritated many researchers.
“What is the worse is the water leakage everywhere else – not just from the tanks. It is leaking out from the basements, it is leaking out from the cracks all over the place. Nobody can measure that.