Slovenia's highest court has ordered a retrial of a high-profile corruption case against a former prime minister who now heads the nation's biggest opposition party.

Janez Jansa, the leader of the right-wing Slovenian Democratic Party, was convicted of bribery in an arms deal with a Finnish company and sentenced in 2013 to two years in prison.

The Constitutional Court, which overturned the conviction, ruled that the evidence presented during the trial was inconclusive. Slovenia's Supreme Court and the Higher Court had previously upheld the conviction.

Jansa has claimed the allegations were politically motivated.

He has served twice as prime minister. His last government fell in 2013 after the state corruption watchdog accused him of failing to declare assets.