The sultan of Brunei has announced a controversial new penal code based on Islamic criminal punishments criticised by UN human rights officials.

"Today... I place my faith in and am grateful to God the almighty to announce that tomorrow, Thursday May 1, will see the enforcement of sharia law phase one, to be followed by the other phases," Sultan Hassanal Bolkiah said.

Islamic law penalties would be introduced over time and would eventually include flogging, amputation and death by stoning for various crimes.

Many members of the Muslim ethnic Malay majority have voiced cautious support for the changes. However, non-Muslim citizens led a rare burst of criticism on social media earlier this year, but largely went silent after the sultan called for a halt.

"Theory states that God's law is harsh and unfair, but God himself has said that his law is indeed fair," the sultan said.

The UN's human rights office recently said it was "deeply concerned", adding that penalties such as stoning were classified under international law as "torture or other cruel, inhuman or degrading treatment or punishment".

Brunei officials have said such cases would require an extremely high burden of proof and judges would have wide discretion to avoid such punishments.

The sultan has warned of pernicious foreign influences such as the internet, and indicated he intends to place more emphasis on Islam.

Nearly 70 percent of Brunei's 400,000 people are Muslim Malays while about 15 percent are non-Muslim ethnic Chinese.