Russian authorities have announced that it will apply for a three-month extension to the detention of the Arctic 30, Greenpeace has said.
They are expected to make the applications in front of a judge at hearings next week.
Thirty people were arrested aboard the ship that was protesting against oil drilling and charged with piracy in September.
Greenpeace International Executive Director Kumi Naidoo said: "We will fiercely resist this absurd attempt to keep those men and women in jail for a crime they did not commit.
"If the authorities succeed then we will appeal and ask for their release as soon as the court can schedule a hearing. This is a farce, it is an outrage that makes a joke of justice. It’s time for the Arctic 30 to come home.”