An international maritime hunt is underway for a missing cargo ship which is thought to be the first hijacked vessel to be sailed through the English Channel in modern times.

The Arctic Sea, a Maltese registered, Latvian-owned ship with a 15-strong Russian crew, vanished with its £1m cargo at the end of July on its way from Finland to Algeria.

British coastguards were the last people known to communicate with the ship on 29 July as it passed along the Channel but it wasn't realised at the time that anything was wrong.

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It is now thought that when the UK's Maritime and Coastguard Agency (MCA) was in radio contact with the ship that the person speaking to them was either a hijacker or a member of the crew with a gun pointed at his head.

The circumstances surrounding its likely hijacking are as puzzling as its current whereabouts. Swedish authorities were told by the Finnish shipping line operating the vessel that on 24 July the Arctic Sea had been boarded by eight to 10 heavily-armed men while it sailed through the Baltic Sea. The crew, three of whom were injured, were tied up and the black-clad and masked men, who purported to be narcotics police, searched the ship.

After 12 hours the intruders left and, supposedly, allowed the vessel to continue on its journey having damaged the communications equipment. But after reaching the Portuguese coast, having sailed along the Channel to get to the Atlantic, the Arctic Sea disappeared from the radar and hasn't been seen since. Its destination had been the Algerian port of Bejaia which it was scheduled to reach on 4 August with its valuable cargo of timber.

Mark Clark, of the MCA, said: "It's highly unusual. We don't know the last time a hijacked vessel sailed the English Channel, it was that long ago. It's very weird, very strange. There's no parallel that we know about that comes this strange.

"We heard from this ship, not knowing it had been hijacked, on 29 July at 5.30 in the morning. Every ship has to report to us if they are on our side of the Channel. They said they had 15 crew on board and they were going from Jacobstad to Bejaia. They were carrying a load of timber.

"It wasn't until later that we had a report from the Zeebrugge police to say it had been hijacked off the coast of Sweden. The contact we had suggested everything was OK on the ship but we don't know if we were talking to a hijacker or a genuine crew member with a gun at his head."

The next time the 3,988 tonne ship, which was built in 1992 and is thought to have been in good condition, was recorded was by a Portuguese coastal patrol aircraft. The timber on the ship belonged to a Finnish-Swedish paper, pulp and timber firm, Stora Enso, which has been trying to get details of the cargo's whereabouts from the vessel's Finnish-based management firm, Solchart Management.

Kari Numminen, of Stora Enso, said: "The shipping line has not told us anything. There is absolutely no information at all. Naturally, wild theories emerge in such a situation."

Swedish, Finnish and Russian authorities are among those investigating the disappearance and the Russian navy is now thought to have dispatched a warship to join the hunt.

"This ship is of interest to very many people," a Spanish coastguard official said. "There are no indications that it would have passed Gibraltar.

"It would be very strange if the ship would have managed to slip through unnoticed."