Kieron Bryan, 29, one of 30 people detained on piracy charges, has been refused bail in a Murmansk court alongside activist Phil Ball.
The Arctic Sunrise was seized by Russian coastguards after a protest against Arctic drilling outside a Gazprom-owned oil rig last month.
John Sauven, executive director of Greenpeace UK, said: "This decision flies in the face of all reason.
"We have offered the Russian courts significant sureties that would guarantee the return of all those accused if and when a trial took place."
The piracy charges the protesters face carry sentences of up to 15 years if they are found guilty, and Russia is considering bringing new charges against the activists.
Authorities said "dual- use equipment" and drugs were found on the Greenpeace ship, and several demonstrators are expected to be charged with more "grave crimes".
Mr Sauven added: "The decision of the court to refuse bail looks increasingly like the Russian authorities are meting out unnecessarily harsh punishment even before any trial.
"In the case of Kieron Bryan, he is a professional journalist, travelling on the Greenpeace ship, witnessing a peaceful protest against oil drilling in the Arctic Ocean.
"He has committed no crime and should be released immediately."
Politicians and celebrities have already demanded Bryan's immediate release.
Actor Jude Law, Blur frontman Damon Albarn and fashion designer Vivienne Westwood have all joined the campaign to free the activists, who had been protesting at the offshore platform in the Pechora Sea.