DUBAI, May 31 (Reuters) - A court in Bahrain has ordered the release of a prominent activist, the state news agency BNA said on Tuesday, citing the well-being of Zainab al-Khawaja's one-year old son who lived in the prison with her.

Khawaja began serving five sentences totalling over three years in March. The charges related to insulting Bahrain's king, according to her lawyer.

She chose to take her son Hadi with her into detention rather than leave him in the care of relatives.

A Bahraini judge ordered her release and that of another mother "on account of their humanitarian situation and for the well-being and interests of their children," the public prosecution statement said.

It was not clear what the charges were against the other mother, named as Irina Bojutova.

Khawaja has been arrested and freed several times since an uprising in 2011 mainly by Bahrain's majority Shi'ite Muslims demanding reforms and a bigger share in government of the Sunni Muslim-led kingdom, and has already served time in prison.

Political unrest in Bahrain has tapered off since 2011 due to a security clampdown on opposition leaders and activists.

A Bahraini appeals court on Monday more than doubled the prison term imposed on the country's most prominent opposition leader, Sheikh Ali Salman, to nine years from four, a ruling that could increase political tensions in the Gulf kingdom.

(Reporting By Noah Browning Editing by Jeremy Gaunt) ((noah.browning@thomsonreuters.com;))