The outspoken Filipino president, Rodrigo Duterte, has vowed to never swear again after he made to a promise to God.

Mr Duterte has become known for his profanity – such as calling Pope Francis a “son of a bitch” and telling US President Barack Obama to “go to hell” for his criticism of the war on drugs which has seen the extrajudicial killings of thousands of people since June.

The controversial politician made the pledge when he returned to his home city of Davao City in the south of the country after a trip to Japan on Thursday.

He explained that while he was flying home he heard a voice threatening to down the plane if he did not stop swearing.

He said heard the voice say ‘If you don't stop epithets, I will bring this plane down now’.

“And I said, 'Who is this?' So, of course, 'it's God’”, the continued.

“So, I promise God to ... not express slang, cuss words and everything.

“So you guys hear me right always because a promise to God is a promise to the Filipino people.”