"The results of the analyses show similarities between the waste on board the Probo Koala in Estonia and the waste delivered by the Probo Koala to Ivory Coast, which caused mass poisoning there," she said.

Last month Trafigura Beheer, the Netherlands-based shipping company that leased the Probo Koala, offloaded more than 500 tonnes of waste -- reportedly a mixture of oil residue and caustic soda used to rinse out the ship's tanks -- from the vessel in Ivory Coast.

A local company charged with disposing of the waste dumped it on open-air rubbish tips in the commercial capital Abidjan, a city of four million people, sparking an environmental catastrophe which has claimed eight lives, seen 69 people hospitalised and triggered 80,000 calls to doctors for medical help.

"There is nothing criminal in a ship requesting to leave the residue from cleaning out its oil tanks in Estonia. But after analysis showed the presence of poisons in the waste residue the Probo Koala wanted to leave in Estonia, we asked the justice ministry to launch an investigation," senior environment ministry official Allan Gromov told AFP.

Environment Minister Villu Reiljan told AFP the crew of the Probo Koala had asked Tuesday to offload waste from the ship in Estonia.

"The analysis samples were taken only yesterday because it was just yesterday that the ship's crew requested permission to leave the waste in Estonia," Reiljan told AFP after visiting Paldiski "to make sure the boat cannot leave the port".

"The Probo Koala expected to leave its toxic waste in Estonia, hoping that our administration would be too weak to prevent this," Reiljan told AFP after visiting the port.

Seeman said the criminal probe would try to find out "whether the Probo Koala, which had asked to leave the cleaning residue in Estonia, knew what the waste contained and wished to hand over residue that contains poisons".

Criminal police were sent to Paldiski port, where the Panamanian-registered ship is docked, to prevent it putting to sea, Seeman said.

Police officers will interview the crew of the ship, more samples will be taken for testing and an additional inspection of the ship will be carried out as part of the probe, Estonian officials said.

The tanker will remain in port as long as needed for the inquiry, the prosecutor's office said.

Ivorian Environment Minister Daniel Aka Ahizi and judge Fatoumata Diakite, who is in charge of a committee in Ivory Coast probing the toxic pollution scandal, have sent separate requests to the Estonian environment ministry to impound the Probo Koala.

Environmental group Greenpeace blockaded the Probo Koala inside Paldiski port, 50 kilometers (30 miles) west of Tallinn, Monday and Tuesday, and urged the Estonian authorities to bar the vessel from putting to sea.

Greenpeace has also filed suit in the Netherlands against Trafigura Beheer in another bid to ensure that the ship remains in Estonia.

But up until Wednesday, the Estonian authorities insisted they had no legal or technical grounds for keeping the ship in the port, even saying that a test conducted on the Probo Koala earlier this month showed it was "clean" of toxic matter.

"When the Estonian authorities kept claiming that the ship was clean and there was nothing toxic on board, they must have been checking the wrong things," Greenpeace activist Satu Pitkanen told AFP from quayside in Paldiski.

Estonian Environment Minister Reiljan had no explanation for why the earlier inspection of the boat gave it a clean bill of health.

"We cannot investigate the entire ship unless we have a good reason to do so," he said. Before the authorities saw the results of the tests carried out on Tuesday, after the Probo Koala asked to dump waste in Estonia, the Baltic state had "no reason to impound the ship", he insisted.

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