Rajiva Wijesinghe said proposed constitutional changes fell short of the promises made by President Maithripala Sirisena before he was elected in January.
"I do not wish to sit among ministers any longer," Wijesinghe said in parliament, calling the government's reform efforts so far "disgraceful".
"I think it is only correct that I cross the floor of the house," said Wijesinghe, who was not in the cabinet.
Sirisena came to power promising to scrap the executive presidency and return the country to a parliamentary democracy.
But Wijesinghe said that did not go far enough, calling for a complete overhaul including the establishment of independent commissions to run the police and public services.
Prime Minister Ranil Wickremesinghe, appointed by Sirisena after his election on January 8, heads a minority government that has the support of several opposition parties.
Wickremesinghe has pledged to dissolve parliament later next month and hold a parliamentary election one year ahead of schedule.
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