Russian investigators have raised the stakes in their battle with Greenpeace, claiming drugs have been found aboard the organisation's ship, the Arctic Sunrise.

They also said a number of Greenpeace activists had put the lives of Russian coastguards at risk with their actions, and announced that new charges are in preparation against some of the 30 people currently in detention. All of those arrested, who are from 18 countries and include six Britons, have already been charged with "piracy as part of an organised group", a charge which carries a sentence of between 10 and 15 years in prison.

The 30 were arrested and their ship seized during a protest against the Prirazlomnaya oil rig, located in the Arctic waters of the Pechora Sea and operated by Russian energy giant Gazprom.

Vladimir Markin, spokesman for Russia's powerful Investigative Committee, said in a statement on Wednesday that criminal investigators and experts were examining documents and equipment seized during a search of the Arctic Sunrise.

"It has already been established that some of the equipment seized has a double use and can be used not only for environmental goals," said Markin, without elaborating.

"Narcotic substances" had also been found on board the boat, which preliminary tests suggested were morphine and poppy straws (opium poppy pods), he added. In a final accusation, Markin said a number of activists had deliberately attacked a coastguard boat during skirmishes during the storming of the Arctic Sunrise, thus "endangering the lives and health" of Russian officials.

"Given the information acquired during the investigation of the criminal case, we will have to correct the charges laid against all [the activists]" said Markin. "It is very clear that a number of them will be charged with committing other serious crimes."

Greenpeace dismissed the drugs allegations as a "fabrication, pure and simple" and said the only drugs on board were locked in a safe and carried for medical purposes as dictated by international maritime law.

"There is a strict policy against recreational drugs on board Greenpeace ships, and any claim that something other than medical supplies were found should be regarded with great suspicion," said the environmental organisation in a statement. The ship was searched by Norwegian authorities with a sniffer dog before leaving for the Russian Arctic, said Greenpeace, and no drugs were found.

Earlier on Tuesday, the executive director of Greenpeace, Kumi Naidoo, published a letter to President Vladimir Putin asking for a meeting with the Russian president. Naidoo said he was ready to travel to Russia and personally guarantee none of the activists would skip bail.

"Were our friends to be released on bail, I offer myself as security against the promise that the 28 Greenpeace International activists will answer for their peaceful protest according to the criminal code of Russia," wrote Naidoo.

• This article was amended on 9 October 2013. A mistranslation resulted in the original article referring to heroin allegedly being found on the ship, instead of morphine. This has been corrected, and the headline and standfirst changed