Polish divers say they have found the wreck of a German second world war ship that may help solve a decades-old mystery about the whereabouts of the Amber Room, an ornate chamber that the Nazis looted from a tsarist palace in Russia.

Decorated with amber and gold, the room was part of the Catherine Palace near St Petersburg. It was last seen in Königsberg, then a Baltic port city in Germany but now the Russian enclave of Kaliningrad.

The Karlsruhe steamer set sail from Königsberg in 1945 with a heavy cargo before Soviet warplanes sank it off the coast of Poland.

Divers from the Baltictech group say they have found the wreck of the Karlsruhe.

“We have been looking for the wreckage since last year when we realised there could be the most interesting, undiscovered story lying at the bottom of the Baltic Sea,” Tomasz Stachura, one of the divers, said in a statement.

“It is practically intact. In its holds we discovered military vehicles, porcelain and many crates with contents still unknown.”

The Karlsruhe had been taking part in Operation Hannibal, one of the largest sea evacuations in history, which helped more than a million German troops and civilians from East Prussia escape the Soviet advance toward the end of the second world war.

Documentation from the time suggests the vessel left Königsberg in a hurry with a large cargo and 1,083 people on board.

“All this, put together, stimulates the human imagination. Finding the German steamer and the crates with contents as yet unknown resting on the bottom of the Baltic Sea may be significant for the whole story,” said Tomasz Zwara, another of the divers.

The Amber Room was constructed in Prussia and then given to Peter the Great of Russia as a present in 1716.

The Nazis dismantled it and took it to Königsberg, from where it disappeared during allied bombing raids on the city. Many believe it was destroyed. Russian craftsmen have constructed a replica of the room in the Catherine Palace.