Khaled Al-Fadalah, who heads the National Democratic Alliance, was found guilty of defaming Sheikh Nasser Al-Mohammad Al-Sabah in remarks at a public rally late last year, according to the source, who said the court also ordered him to pay a fine of 150 Kuwaiti dinars ($516).

The world's fourth largest oil exporter generally allows more criticism of public officials in the press than other Gulf Arab states, but its ruler is protected from criticism by the constitution. Many writers have been fined or imprisoned for defamation.

The ruling, which can be appealed, comes days after an opposition Kuwaiti blogger and journalist was released on bail as his trial continues on charges of insulting Kuwait's emir and inciting against the government.

Mohammad Abdul-Kader Al-Jassem, whose case has drawn US concern, faces up to 18 years in jail if he is convicted.