Two senior transport inspectors have been arrested in Russia over the sinking of the Bulgaria cruise ship, which killed 122 people in July 2011.

Russia's Investigation Committee has detained the two transport inspectors Irek Timergazeev and Vladislav Semyonov, senior river fleet inspectors from Volga department of the country's transport watchdog Rostransnadzor, a spokesman said on Tuesday, as cited by RIA Novosti.

The Bulgaria cruise ship sank during a storm in the Volga River in the Republic of Tatarstan on July 10, killing 122 people. There were over 200 passengers on board the heavily overloaded vessel when it went down.

Irek Timergazeev and Vladislav Semyonov are said to have allowed the AgroRechTour company to carry passengers on the Bulgaria, despite the vessel lacking the appropriate license.

"The illegal actions of the Rostransnadzor employees are directly related to their grave consequences - the death of the 122 people," spokesman Vladimir Markin said.

Two people have already been charged over the sinking of the ship. Svetlana Inyakina, the general director of the company that rented the cruise boat, and river fleet inspector Yakov Ivashov, who certified that the Bulgaria was fit to sail, face prison terms if found guilty.

The arrests come a day after Russia's transportation watchdog attributed the July 10 cruise ship accident on the Volga River to safety violations and a poorly trained crew.

"The ship sank because it lost balance in the storm after water flowed into open portholes," the transport watchdog said in its investigative report, adding that this had made the ship lean to the right and eventually sink.

Safety regulations require that all portholes, including the ones in passenger rooms, be closed before a vessel starts moving.

"During the inspection of the sunken ship, divers discovered that 27 portholes were opened on the left side of the vessel and 11 on the right side," the Federal Service for the Oversight of Transportation said on Monday.

Shortly after the tragedy, media reports showed that the Bulgaria riverboat, built in 1955, had last undergone an overhaul 30 years ago and had no license to carry passengers.