JAKARTA (Reuters) - Indonesian President Joko Widodo and Malaysian Prime Minister Mahatir Mohamad have signed a joint letter of objection to the European Union over its plan to phase out the use of palm oil in renewable fuel, an Indonesian official said.

The letter was sent to the EU over the weekend, said Luhut Pandjaitan, coordinating minister for maritime affairs who also oversees natural resources issues. He declined to disclose content of the letter.

Indonesia and Malaysia, the world's top producers of the vegetable oil, have both threatened a World Trade Organization challenge against the EU over its plan to phase out the use of palm oil by 2030 in renewable transport fuel.

(Reporting by Wilda Asmarini; Writing by Fransiska Nangoy; Editing by Michael Perry)