Japan has vowed to press on and kill 50 humpback whales later this year in defiance of conservationists and anti-whaling nations.

Britain joined New Zealand, Australia and other "like-minded" nations to condemn the plan in Anchorage, Alaska, where the annual conference of the International Whaling Commission opened on Monday.

"This will really adversely affect the image of Japan in our countries," said New Zealand's Environment minister, Chris Carter.

Japanese whaling ships intend to kill the humpbacks in the Southern Ocean whale sanctuary as part of its "scientific whaling" programme. The ships will also hunt hundreds of minke, sei, sperm and fin whales.

The humpback is classed by most environmentalists as one of the planet's more imperiled species, but not by Japan's Fisheries Agency. "We don't see it as endangered," said Joji Morishita, Tokyo's alternate IWC commissioner. "Our surveys suggest that in some areas humpback stocks are increasing."

Tokyo has hinted at a possible deal to limit the size of the humpback hunt in return for concessions on commercial whaling. Conservationists suspect that Japan might use the plan to kill one of the planet's more beloved mammals as a bargaining chip in its efforts to secure a return to small-scale commercial whaling by its coastal communities.

But the overture was rejected by Britain's Biodiversity minister, Barry Gardiner, who said that the humpack issue was not "a matter of horse trading and negotiations".

Conservationists say that scientific whaling is illegal and that the IWC should stop it, despite threats from the pro-whaling nations that blocking compromise will push them into a corner.

Officially, 29,000 whales have been slaughtered since a worldwide moratorium on commercial whaling in 1986, most by Japan, Norway and Iceland. Many more are snagged in fishing nets, something conservationists say causes 300,000 deaths a year. Others die from ingesting oil, chemicals and plastic in congested sea lanes.

The meat harvested is increasingly too polluted to eat. Iceland's commercial whaling campaign has been stalled ­ after mercury was found in whale carcasses.

"With so many other factors impacting whale populations, it is incredible that the IWC is still entertaining the idea of commercial whaling, " said Junichi Sato of Greenpeace Japan. He wants a commitment this year to switch the IWC into a "body that works for the whales and not the whalers".

Pro-whaling nations see the IWC in opposite terms, clinging to what they say is its original mission, the managed, sustainable use of whale resources. Japan has never accepted the conservationist takeover of the IWC and has waged a $750m (£380m) campaign to swing the organisation back to support commercial whaling. Last year, it won a narrow vote for the first time in 25 years, a symbolic victory that stunned environmentalists.