Authorities in Russia have claimed that they had found illegal drugs on the ship of 30 Greenpeace crew members detained on piracy charges, promising additional charges against several defendants.

In a search of the Greenpeace ship Arctic Sunrise, investigators confiscated “narcotic substances, presumably poppy straw and morphine” and are determining the origin and purpose of the drugs, Vladimir Markin, the spokesman of Russia’s powerful Investigative Committee, said.

Poppy straw is illegal in Russia, while morphine is only legal only on a doctor’s orders.

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Russian authorities seized the Arctic Sunrise in a commando-style raid last month, towing it to Murmansk and later charging 28 activists and crew members and two freelance journalists with piracy, which carries a prison sentence of 10 to 15 years. Six Britons are among those being detained.

Captain Peter Willcox was also charged with resisting the authorities, an administrative violation.

The 30 defendants are being held in pre-trial detention centres around Murmansk, and three defendants’ requests for bail were rejected on Tuesday. Greenpeace lawyers have said they will file a case with the European Court of Human Rights over the conditions faced by their clients in detainment.

The Investigative Committee’s statement suggested the piracy charges may be cancelled and replaced with other criminal charges. President Vladimir Putin previously said that the activists are “obviously” not pirates, but investigators nonetheless pressed ahead with piracy charges.

“Given the information gathered during the course of investigating this criminal case, the charge that has already been brought against everyone is expected to be changed. Meanwhile, it’s obvious to investigators that several defendants will be charged with committing other serious crimes,” Mr Markin said.

It was not immediately clear what punishment possible drug charges could entail, but in previous cases, people convicted of selling poppy straw in Russia have been sentenced to five or more years in prison.

Greenpeace and other commentators criticised the possible narcotics charges as ridiculous.

“We can only assume the Russian authorities are referring to the medical supplies that our ships are obliged to carry under maritime law,” Greenpeace said in a statement.

“In Russia, they ‘find’ narcotics on those being investigated when all other evidence falls through,” tweeted popular blogger and opposition activist Oleg Kozyrev.

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Mr Markin repeated investigators’ claims that the defendants endangered the lives of government employees, saying they are determining those “who deliberately rammed coast guard boats”. He added that some equipment confiscated from the ship “can be used not only for environmental goals” and would be examined in court.