Japan’s hopes of resuming its whale hunts in the Southern Ocean have suffered a setback after International Whaling Committee experts said its latest plan offered no scientific justification for the slaughter.

The IWC panel said Japan’s revised programme, known as Newrep-A, did not contain enough information for experts to determine whether Japan needed to kill whales to fulfil two key objectives: calculating the size of populations necessary for a return to sustainable commercial hunting, and gaining a better understanding of the Antarctic marine ecosystem.

“With the information presented in the proposal, the panel was not able to determine whether lethal sampling is necessary to achieve the two major objectives,” the IWC experts’ report said. “Therefore the current proposal does not demonstrate the need for lethal sampling to achieve those objectives.”

Japanese officials have been working on a revised whaling programme since last year when the international court of justice in The Hague ordered an immediate halt to its Antarctic hunts after concluding that they were not, as Japan had claimed, being conducted for scientific research.

The UN court’s ruling was in response to a landmark legal challenge to the Southern Ocean hunts by Australia, which claimed Japan was using science as a cover for commercial whaling.

Under the moratorium on commercial whaling Japan is allowed to sell meat from the “scientific” hunts on the open market, although consumption has fallen dramatically since the postwar years when it was a rare source of protein.

Tokyo hoped that its revised plan, involving the killing of fewer whales, would pave the way for the resumption of the Antarctic hunts, possibly by the end of this year.

Its whaling fleet recently returned from the Southern Ocean, although it had not planned to kill any whales, in accordance with the ICJ ruling.

“The ICJ ruling ensured that for the first season in more than a century whales in the southern hemisphere were not hunted for commercial purposes,” said Patrick Ramage, global whale programme director for the International Fund for Animal Welfare.

“It is disappointing … that Japan’s fisheries bureaucrats would defy the world’s highest court and try to restart illegal whaling in the Southern Ocean.”

In its reworked plan Japan proposed an annual cull of up to 333 minke whales over the next 12 years, down from more than 900 a year previously. The total cull over that period would reach 3,996 whales, compared with the 13,000 whales it has killed since the IWC ban on commercial whaling came into effect in 1987.

Japan has long claimed that it needs to conduct “lethal research” to better understand whale populations’ migratory, feeding and reproduction habits with a view to a return to commercial whaling. It argues that many whale species, including minke, are not endangered.

Japanese officials said they would provide more information before the IWC’s scientific committee meets in San Diego next month. “I believe that we’ll move forward with the aim of resuming whaling around the end of the year,” the country’s commissioner to the IWC, Joji Morishita, told reporters, although he did not rule out changes to the proposal.

Morishita said Japan took the panel’s report seriously but added: “They haven’t unilaterally said that it’s no good; neither have they come out on the other side with ‘Go ahead, do whatever research you want to do.’”

Environmental campaigners welcomed the IWC panel’s decision. “[The findings] reiterate and underline the concern of the international community: you don’t need to kill whales in order to study them,” said Claire Bass, UK director of the Humane Society International.

“It has long been clear that Japan’s large-scale whaling operations are driven by politicians, not scientists, and serve no useful conservation or scientific need. This latest report from the IWC review panel essentially sends Japan back to the drawing board as it has failed to make a case for the need to kill whales in the name of science.”