The Sultan of Brunei has stripped his daughter-in-law, Pengiran Raabi'atul Adawiyyah, of her royal titles due to 'unbecoming behaviour'.

Hassanal Bolkiah, the world's longest-reigning monarch, issued the decree after the 33-year-old 'tarnished the monarchy's good name', according to a statement.

Pengiran joined the Brunei royal family when she wed Prince Abdul Malik, fourth in line to the Brunei throne, in a glittering ten-day ceremony in 2015.

The couple have since welcomed four children together, whose titles remain unaffected: Princess Muthee'ah, 10, Princess Fathiyyah, eight, Princess Khaalishah, six, and Princess Nabeelah, 16 months.

Prince Abdul is the Sultan's second son from his first marriage to Queen Saleha. The Sultan has 12 children with three wives.

While the palace hasn't revealed the specific actions that caused Pengiran to lose her titles, according to reports by Malaysia's New Straits Times, the Office of the Grand Chamberlain confirmed she is no longer of the royal family and stated:

'The revocation was carried out following conduct by Pengiran Raabi'atul Adawiyyah that was deemed unbecoming of a spouse of a member of the royal family.

'Her actions were also found to have tarnished the monarchy's good name and demonstrated a lack of respect towards Sultan Hassanal Bolkiah.'

In addition to being the country's Sultan and absolute monarch, 80-year-old Hassanal Bolkiah has also been the Prime Minister of Brunei since the country gained independence from the United Kingdom in 1984.

His reign has also been mired in controversy, and he's faced criticism of his country's human rights record.

Despite this, Bolkiah appeared to have enjoyed a close relationship with the late Queen Elizabeth.

Like his father before him, he was knighted by Her Majesty, and the pair met on several occasions during their reigns – in both London and Bandar Seri Begawan.

Unlike the United Kingdom's head of state, Bolkiah enjoys absolute power in Brunei. Under the country's 1959 constitution, the head has full executive authority – including emergency powers.

In 2006, he was reported to have amended the country's constitution to make himself infallible.

As Prime Minister, he is also the head of government – and currently holds the positions of Minister of Defence, Minister of Foreign Affairs and Minister of Finance. He also controls the country's police force.

He is also Head of Religion, and Islam is the country’s official religion. In 2014, he advocated for the adoption of Islamic sharia penalties, which include death by stoning, the severing of limbs and flogging for some crimes.

Such crimes include abortions, adultery and same-sex sexual acts.

His push in 2014 raised alarm bells among human rights advocates, and celebrities – including George Clooney and Elton John – called for a boycott of hotels associated with the Sultan, notably the Dorchester Collection, which has hotels in the US and Europe.

Educational institutions with connections to the Sultan, such as Oxford and King's College London, were also urged to strip his honorary degrees.

As Prime Minister, the Sultan has spearheaded anti-LGBT legislation. As of April 2019, homosexuality and adultery are punishable by death by stoning.

The rule outlawed gay sex, which had to be independently witnessed in order to secure a conviction.