Hussein Radjabu fell out with the president last year
Burundi's Supreme Court has sentenced one of the country's most powerful politicians, Hussein Radjabu, to 13 years in prison for subversion.
The former ruling party chairman was accused of plotting an armed rebellion and insulting the president by referring to him as an "empty bottle".
Reporters say the move will create an even deeper rift in the ex-rebel Forces for the Defence of Democracy (FDD).
The party won polls in 2005 ending 12 years of civil conflict.
Mr Radjabu's differences with President Pierre Nkurunzinza came to the fore in February last year after delegates at a special congress of the ruling party replaced him as chairman.
He was then arrested in April 2007.
The BBC's Prime Ndikumagenge in the capital, Bujumbura, says his sentence is expected to anger the minority Muslim community, of which Mr Radjabu is a leading member.
It will also anger many in the national assembly, where a third of the MPs are believed to support him, he says.
President Nkurunziza has carried out two cabinet reshuffles since the fall-out, replacing most of Mr Radjabu's supporters, including the vice-president, from key government positions.
Divisions within the ruling party last year prompted fears of renewed violence.
Since independence in 1961, Burundi has been plagued by tension between the dominant Tutsi minority and Hutu majority.
More than 300,000 people died in the war sparked in 1993 by the assassination of Burundi's first Hutu head of state and democratically elected president, Melchior Ndadaye.
Hutu rebels, such as the FDD, took up arms before South Africa brokered a power-sharing deal in 2001.