Tonga's king, George Tupou V, is to give up much of the near-absolute power his family has held for generations.
He is to surrender his role in day-to-day governmental affairs and be guided by the PM, his spokesman has said.
The monarch - who took over from his father, Tupou IV, in September 2006 - is being formally enthroned on Friday.
The palace had long pledged democratic reforms, but progress has been slow and public dissatisfaction was growing, correspondents say.
George V, 60, is making the changes to prepare the monarchy for 2010, when most of the new parliament will be elected, said Hon Fielakepa, the Lord Chamberlain and royal spokesman.
"The Sovereign of the only Polynesian kingdom... is voluntarily surrendering his powers to meet the democratic aspirations of many of his people," he said.
Last week, the government said the king had completed the sale of his ownership of state assets which had contributed to much of the royal family's wealth.
Tonga is one of a handful of countries in the world where the monarch has a hand in the daily running of government.
But pressure for change has been building in the South Pacific nation.
Unrest boiled over into riots in 2006 at a pro-democracy rally in the capital, Nuku'alofa, that left eight people dead and destroyed much of the city. George V's coronation was delayed as a result.