The Chief Pleas will become an elected chamber for the first time
The UK Privy Council has approved proposed changes to the governing body of a Channel Island which still operates a feudal system of government.
Sark's ruling body, the Chief Pleas, breaches the European Convention of Human Rights because landowners have got a seat automatically for 450 years.
The Chief Pleas had already approved new reforms for an elected chamber.
A lawyer for millionaires Sir David and Sir Frederick Barclay, who own nearby Brecqhou island, said they will appeal.
Advocate Gordon Dawes said the approval will be disputed in the High Court in London as the reforms do not address the role of the seigneur and seneschal.
The seigneur is the head of the Chief Pleas and retains feudal rights and the seneschal is president of the Chief Pleas and head of the judiciary.
The Barclay brothers say their roles are incompatible with the reforms.
But Sark's Seneschal, Reg Guille, said the decision to change the system of government reflected the wishes of most of the island's people.
Sark has been governed by a mix of landowners and elected people's deputies since the 1600s.
Owners of the island's 40 tenements (divisions of land) currently have an automatic seat in the Chief Pleas, with islanders choosing 12 deputies.
They will be replaced by the new 28-member chamber, which was approved following a referendum for islanders voting for democracy.
The Privy Council's approval enables Sark's judiciary and parliament to be significantly modernised.
A spokeswoman for the Privy Council said: "The meeting has concluded and the Sark laws have been approved."
The move comes following pressure on Sark to change its feudal system to comply with European human rights laws and other international obligations.
The presiding officer of the Chief Pleas, Lt Col Reginald Guille, the Seneschal of Sark, said the changes would help bring the island's judiciary and government in line with the 21st Century.
He said: "These moves are intended to be a step away from a feudalist system, but at the same time still keeping some aspects of that system in place.
"It will see huge changes to our judiciary and government, making both more modern."
The self-sufficient, car-free island is just 3 miles (4.8km) long and 1.5 miles (2.4km) wide and has a resident population of about 600.
The only forms of transport permitted are horse-drawn vehicles, bicycles, tractors and battery-powered buggies.
Lt Col Guille described life on the island as "idyllic", saying: "It's a very quiet and peaceful place.
"We are a self-sufficient, close-knit little community and we just like to get on with life away from the public eye."
The government there can directly trace its roots back to Queen Elizabeth I, who once granted the ruling "Seigner" a fief on the tiny Channel Island.
The unelected descendents of 40 families brought in to colonise Sark, after the French abandoned it in 1553, have governed life on the island ever since.
But its feudal system of government started coming under pressure in 2000 in the light of human rights laws.
Two proposals for reform were rejected in 2005 and 2007 until the island's historic referendum.