The International Whaling Commission's panel of experts opposes Japan's proposal for its scientific research whaling programme which it says does not demonstrate a need for killing whales.
It has asked Tokyo for more information to justify the killings.
The International Court of Justice last year ruled that Japan's decades-old whale hunt in the Southern Ocean should stop, prompting Tokyo to cancel the bulk of its whaling for the 2014 - 2015 season and submit a scaled-down plan to the International Whaling Commission (IWC) for future hunts.
Japan wants to slaughter 330 minke whales annually for 12 years, starting later this year.
It submitted a revised whaling plan to the commission's independent review panel.
Patrick Ramage from the International Fund for Animal Welfare said the IWC panel found there was no scientific justification for continuing lethal research whaling.
"The grounds and reasons provided by Japan were inadequate, that they had in fact not shown that there was any reason to do lethal research in the Southern Ocean."
Instead he said Japan should be encouraged to join Australia, New Zealand, the UK, the United States and many other countries "that are pursuing 21st century science".
"There's far more to learn from observing whales - living whales in the marine environment - than by killing them and chopping them up and examining their bits."
The report will go before the commission's full scientific committee next month.