Scientists in Japan claim that their country's controversial whaling programme, which has killed thousands of minke whales since the late 1980s, has established that the animals have lost significant amounts of blubber. Measurements taken from more than 4,500 slaughtered minkes show they are getting thinner at a worrying speed, the researchers say.

The team from the Institute of Cetacean Research in Tokyo, set up to analyse the results of the scientific whaling programme, says its study offers the first evidence that global warming could be harming whales, because it restricts food supplies. They say the discovery could only have been made by killing the animals.

Crucially for the Japanese, the results have been published in a mainstream western scientific journal, Polar Biology. Campaigners say publication could offer scientific whaling a veneer of respectability, and bolster Japan's efforts to hunt more whales.

They fear Japan could use the results to support efforts to hunt endangered humpback whales for the first time in 50 years. The study claims the recovering humpback population in the Southern Ocean could also be hurting the minkes because of "interference" between the two species as they compete for food.

Campaigners and politicians say Japan's scientific whaling programme is commercial whaling by another name and is unethical and unnecessary.

Lars Walloe, a Norwegian whale expert at the University of Oslo, who helped the Japanese team analyse the data, and an author of the study, said: "This is a big change in blubber and if it continues it could make it more difficult for the whales to survive. It indicates there have been some big changes in their ecosystem."

Whales rely on blubber for energy and insulation. The shift could already be making it more difficult to reproduce, Walloe said. "I don't think you could measure this by other [non lethal] means."

He said the Japanese findings, and their publication, had been unpopular among scientists from nations opposed to whaling, including Britain. Two journals refused to print the findings before they were accepted by Polar Biology, which published them online last month. Walloe, who says he does not support the ban on commercial whaling, claimed that the journals that turned down the study did so for political, not scientific, reasons.

The findings are the most high profile of the Japanese scientific whaling programme so far.

Mark Simmonds, director of science at the Whale and Dolphin Conservation Society, said: "Lots of dead bodies will provide robust data, so if you kill lots of whales then you will get some information. The question is whether the benefits outweigh the number of whales killed and how they were killed. Scientific whaling is not about science, and there is no pressing conservation need that requires it."