The Sultan of Brunei, one of the world’s wealthiest rulers and a close ally of Britain, will this week oversee his country’s transition to a system of Islamic law with punishments that include flogging, the dismemberment of limbs and stoning to death.

The 67-year-old absolute monarch declared last year that he wanted to introduce a full sharia system in his oil-rich nation and warned critics who took to social media sites to complain that they could be prosecuted using the new laws.

The decision to introduce sharia and reintroduce the death penalty has been condemned by NGOs and legal rights campaigners, who say the new rules will breach international laws. It has also triggered alarm among some of Brunei’s non-Muslim communities, who will also be subject to some of the rulings.

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The development could put pressure on Britain to rethink its close relationship with Brunei, a former colony. A British regiment based in the country – the last surviving UK regiment stationed in East Asia – is paid for entirely by the Sultan.

In a letter to Sultan Hassanal Bolkiah, the International Commission of Jurists (ICJ) said it deplored the new rules, adding that, if implemented, they would lead to serious human rights violations. “Brunei has not implemented the death penalty for years, so it came as quite a surprise that the new law has reintroduced it,” said the ICJ’s Emerlynne Gil.

Brunei is two-thirds Muslim and has long implemented some sharia, mainly for civil matters such as marriage. But last year the Sultan, who is said to be worth £24bn and lives in a 1,788-room palace, announced a plan to introduce full Islamic law.

Offences include insulting the Prophet Mohamed, drinking alcohol, getting pregnant outside of marriage and “sodomy”. The latter will be punishable by stoning.

“It is because of our need that Allah the Almighty, in all his generosity, has created laws for us, so that we can utilise them to obtain justice,” the Sultan said at the time.

It is unclear precisely what is motivating the Sultan, who also serves as the country’s prime minister and assumed the throne in 1967. But in a speech in February to mark the country’s National Day holiday, he claimed the system of an absolute Islamic monarch acted as a “strong and effective firewall” against the challenges of globalisation. He referred specifically to the internet.

He claimed that there were those, both in and outside Brunei, which last year chaired the Association of South-East Asian Nations (Asean), who had been challenging his plans and who wanted to see “internal turmoil”. He added: “These parties, it seems, have attempted to mock the king, the Islamic scholars and sharia. They are using the new media, such as blogs, WhatsApp and so on, which are not just accessed by locals but also by those overseas.”

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The speech by the Sultan – who for many years was involved in a high-profile legal battle with his brother, a playboy accused of misappropriating £9bn of government assets and who reportedly owned a yacht called Tits – has had the impact of silencing many who might publicly speak out against the move.

Yet there are concerns, especially among the minority communities. There are around 30,000 Filipino citizens in Brunei, many of them Catholic, and the Philippine ambassador to Brunei, Nestor Ochoa, recently held a meeting at which he warned his countrymen about the implications of the new laws.

Father Robert Leong, a Catholic priest in Brunei, said there were concerns that baptisms of newborn babies could breach the new rules, which prohibit the “propagation of religion other than Islam to a Muslim or a person having no religion”. He said that the law was being introduced in three phases, with the harshest punishments, including the death penalty, being phased in over two years from Tuesday. “There will be no baptisms. There is not a lot we can do about it. We will have to wait and see what happens,” he said.

Britain granted independence to Brunei in 1984, but has maintained a close relationship with the country. A 1,000-strong regiment of the British Army, the Royal Gurkha Rifles, has been located there since the late 1950s and in 1962 stepped in to quell a rebellion against the Sultan’s father. The regiment is paid for by the Sultan. The British Army also runs a jungle warfare training school in the small nation. A government spokesperson said: “Ministry of Defence discussions are ongoing with the Bruneian authorities to clarify any impact on UK forces.”

Royal Dutch Shell, an Anglo-Dutch multinational, also runs a major operation there as a joint venture with the Brunei government.

A briefing document published last year about defence and security opportunities in Brunei by the UK Department for Business, Innovation and Skills said Brunei invested “a significant proportion of the country’s wealth through the City of London”. It said the British Armed Forces garrison was a linchpin of UK-Brunei relations.

“The Government’s goal is to retain a dominant position in these key areas, and to maximise our share of influence as Brunei diversifies its economy and puts increasing emphasis on regional partners like Asean and China,” it said. “As it does so, Brunei will also provide a UK-friendly window into the key growth area of South-east Asia.”

The Sultan has been married three times. He remains married to his first wife, but he divorced his second, a one-time airline stewardess, in 2003 after 21 years. He divorced his third wife, a former TV reporter, in 2010 after five years. Both ex-wives were stripped of their royal titles.

Stories of his wealth abound. It was reported that, while playing polo with Prince Charles on one occasion, he had his boots delivered by helicopter to the polo field.

The Brunei government did not respond to queries and the Brunei High Commission in London failed to answer questions from The IoS. However, earlier this year, Brunei’s most senior Muslim cleric claimed that those criticising the new rules did not understand them, according to a report in The Brunei Times.

Dr Ustaz Hj Awg Abdul Aziz Juned said in a lecture in London: “Not even a day after the law was announced, human rights groups on social media commented that the steps taken by the Brunei government to implement the law was out of date and not modern.”

Sharia explained

What is sharia?

Sharia is the Islamic legal system that derives from the Koran, the example of the life of the Prophet Mohamed and “fatwas”, which are the rulings of Islamic scholars. Different schools of thought exist, resulting in different interpretations.

What does it cover?

While Western law confines itself predominantly to crime and civil matters, sharia is a guide to help Muslims understand how they should lead every aspect of their lives. This ranges from deciding whether to enter a bar with someone wanting to drink alcohol to the punishments for theft or for criticising the Koran. Its treatment of women is particularly controversial. Judgements have banned the holding of property once married, enabled beatings for insubordination, and required a husband’s consent to divorce.

Where is it used?

Mauritania, Sudan, Afghanistan, Iran, Iraq, Maldives, Pakistan, Qatar, Saudi Arabia, Yemen and Nigeria all apply sharia. Some states, including UAE, Jordan and Egypt, use some form of sharia in their judicial system.