A Russian court has refused bail to freelance videographer Kieron Bryan and Greenpeace activist Phillip Ball during a hearing in Murmansk.
The court ruled that Bryan, from London, and Ball, from Oxfordshire, should remain in pre-trial detention until 24 November.
Greenpeace has called for their immediate release saying the "decision flies in the face of all reason".
The pair were among six Britons on board the Arctic Sunrise boat. Some 30 people have now been detained on piracy charges since the incident on September 18.
The Arctic Sunrise was seized by Russian coastguards following a protest over 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".
Russian investigators said they are trying to identify people whose inflated boats were said to be standing in the way of Russian coast guards, "threatening life and safety of an official".
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 the immediate release of the freelance videographer.
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.
Greenpeace denies any wrongdoing and described the charge as "absurd and outrageous".