The Sultan of Brunei. [Photo/Xinhua]

Brunei has banned Christmas, warning that anyone found celebrating will face a fine of up to $20,000, up to five years in prison, or both.

The Sultan of Brunei (pictured) made a declaration reminding citizens that celebrating the Christian festival in public is banned, including sending festive greetings and the wearing of Santa Claus hats.

The ban was introduced last year after fears that celebrating Christmas "excessively and openly" could lead the Muslim population astray.

At least 65% of Brunei's 420,000-strong population is Muslim.

There is a loophole to the ban: Christians can celebrate the festival but must first let the authorities know, and then enjoy the festivities in their own communities on the condition that the celebrations are not disclosed to Muslims.

Religious leaders support the ban as some interpretations of Islam prohibit imitating another faith, while the Ministry of Religious Affairs has reportedly sent agents to check that local businesses do not have Christmas decorations up.

Not everyone is happy about the ban on Christmas; a social media campaign, run under the hashtag #MyTreedom has seen some Brunei residents post photos of their celebrations online.