The newly amended Crown Property Act, announced in a palace statement on Sunday, replaces three laws dating back to as early as 1936, and is the first amendment to legislation concerning crown property in decades.
It is the latest change to give greater authority to the king, who has shown himself increasingly assertive since ascending the throne in December following the death in October of his father, King Bhumibol Adulyadej, who ruled for seven decades.
The exact size of the Crown Property Bureau is not made public, but recent estimates have run to more than $30 billion through its holdings in real estate and other investments.
The amendment places the management of crown property under the direct supervision of the king. It states that the bureau's properties, in addition to the king's private properties, will be managed "at His Majesty's discretion".
It also states that the monarch can assign the Crown Property Bureau, any individual or agency to manage the properties and assets.
It gives the king sole power to appoint a board of directors to manage crown property and appoint its chairperson, a position previously held by whoever was finance minister.
Rich-list publisher Forbes once described the former king as the world's richest royal, but was rebuked by the Thai embassy in Washington, which said assets in the Crown Property Bureau were not his property but held "in trust for the nation".
A distinction was made between the king's private property and crown property, which belongs to the monarchy as an institution and was under the direct management of the Crown Property Bureau.
A member of the Crown Property Bureau declined to comment on the law when contacted by Reuters.
The bureau holds a share of Siam Commercial Bank Pcl, Thailand's third-biggest commercial bank. It is also the largest shareholder of Siam Cement Pcl, Thailand's largest industrial conglomerate.
Those two stakes are together worth more than $9 billion.
The amendment to the law follows the transfer in April of various royal agencies from the government to the king's supervision.
Thailand is a constitutional monarchy, but King Bhumibol wielded great influence and his son has shown himself to be assertive.
(Reporting by Bangkok Bureau; Editing by Amy Sawitta Lefevre, Matthew Tostevin, Robert Birsel)
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