Greenpeace said today it had decided not to harass Japanese whalers in the Antarctic this year so it can focus on clearing two activists being held for allegedly stealing whale meat it says was bound for the black market.

The group said it would plough its resources into building an anti-whaling coalition in Japan and defending Junichi Sato and Toru Suzuki against the theft charges. If convicted, they could be sent to prison for 10 years.

"Our actions have taken the plight of the whales to the top of the political agenda," Jun Hoshikawa, Greenpeace Japan's executive director, said in a statement. "But if we are to bring this hunt to an end, we have to bring change in Tokyo."

Greenpeace denied it had bowed to pressure from the Japanese authorities. "The key country we need to influence is Japan, and we want to do that inside Japan," Greenpeace spokesman Dave Walsh told the Guardian. "We do have an international impact, but there's not a lot of point in preaching to the converted.

"We have saved whales every year we've been in the Antarctic, but we want to end whaling for good and we can't do that if all our resources and energy are focused on the Southern Ocean."

However, the whaling fleet, which is due to leave for the Southern Ocean later this month, will not be able to carry out its cull unhampered.

Sea Shepherd, a marine conservation group that advocates direct action, has vowed to disrupt the fleet's attempts to slaughter whales as part of Japan's controversial scientific whaling programme.

"We intend to be more aggressive and even more relentless," Peter Hammarstedt, a Sea Shepherd activist, said in a statement. "We intend to make sure no whales are caught on our watch."

Last year Japan's whalers had planned to catch 850 minke whales but returned this spring with 551 after being frustrated by Greenpeace and Sea Shepherd activists.

Although commercial whaling was banned in 1986, Japan is permitted to catch whales for "lethal research" into the mammals' migratory and other habits in anticipation of a return to sustainable commercial culls.

Sato and Suzuki were arrested in June after intercepting 23kg of whale meat they claimed had been pilfered by crew from the factory ship the Nisshin Maru and sold on Japan's lucrative black market. They will go on trial early next year.

"Junichi and Toru's trial is politically motivated," said Sara Holden, Greenpeace's whale campaign coordinator. "If the Japanese government wants to make political prisoners out of people who oppose whaling, then they are going to have to take a lot more prisoners."

The group said 250,000 people had emailed the Japanese prime minister, Taro Aso, demanding that the charges be dropped.