The ship sailed through the English Channel by pirates was last night feared to be carrying a secret cargo of arms for terrorists.
Maritime authorities have dismissed theories that the Arctic Sea was snatched for its £1 million cargo of timber.
Experts now believe it has been loaded with a secret cache of weapons.
The Arctic Sea spent two weeks at a shipyard in Kaliningrad - a Russian enclave notorious for smuggling - before loading timber in Finland. Russian maritime expert Mikhail Voitenko said: "The only sensible answer is that the ship was loaded secretly with something we don't know about.
"It cannot be drugs or illegal cargo, it is something much more dangerous."
Russia's entire Atlantic fleet has been scrambled to track down the Maltese-flagged vessel and its 15 Russian crew. Its last radio message, received two weeks ago, was to the UK Coastguard in the Channel.
The ship had been boarded days earlier in the Baltic Sea by a gang posing as cops in the first piracy in European waters for hundreds of years.
The ship was due to arrive in Algeria on August 4, but is now thought to be somewhere in the Atlantic.
n.parker@the-sun.co.uk 
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