JAKARTA (Reuters) - Indonesia is set to pass a revision to its constitutional court law, legislators have confirmed, ushering in changes that legal experts say will threaten the independence of the judiciary in the world's third-largest democracy.

The proposed amendment will reduce the tenure of justices from 15 to 10 years, and require those who have served more than five years to obtain approval from appointing institutions to continue their tenure.

Constitutional Court judges are appointed by the president, parliament and Supreme Court.

The revision also includes changes to the composition of the ethics council that oversees the court, proposing the addition of the president, and members of the parliament and supreme court.

The amendments have sparked strong criticism from some legal experts.

"These changes will ruin the independence of the constitutional court," said Bivitri Susanti, from Indonesia's Jentera School of Law,

"The intention is really to put the judiciary, the constitutional court, under the control of the House of Representatives and the president."

The parliament and government have agreed to the bill and will ratify it at the plenary session, Adies Kadir, deputy head of a parliamentary commission responsible for drafting it, said in statement posted on the parliament's website.

Sarifuddin Sudding, another member of that commission, told Reuters the bill had been agreed upon, but a date for it to be ratified had not been set.

The next plenary session is scheduled for Monday, May 20.

If passed, the revised law would apply to three judges who provided dissenting opinions in several high-profile cases, including challenges to this February's presidential election.

The amendment comes amid criticism of a democratic backslide in Indonesia and as the court has been in the spotlight, particularly after a controversial ruling last October that enabled the son of President Joko Widodo to contest the vote.

At the time, the chief justice of that court was the president's brother-in-law, who was later reprimanded by the court's ethics council for allowing intervention from an unspecified "external party".

(Reporting by Kate Lamb and Ananda Teresia; Writing by Kate Lamb; Editing by Kim Coghill)

By Kate Lamb and Ananda Teresia