Ruak has repeatedly failed to secure passage of a budget for 2020 after the largest party in his coalition,the National Congress for Timorese Reconstruction (CNRT) party of independence hero Xanana Gusmao,withdrew support.
"I have sent a[resignation] letter to the President,"Ruak said.
He had been backed by a three-party coalition,the Alliance of Change for Progress (AMP),that won 34 of the 65 seats up for grabs in May 2018,the country's fifth election since independence from Indonesia in 2002.
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But there has been occasional political deadlock and growing tension after the President,who belongs to the opposition Fretilin party,rejected some ministers proposed by Gusmao,over accusations of corruption.
On Saturday,Gusmao,East Timor's first president and a former prime minister,announced a new six-party coalition controlling 34 seats without Ruak's party. Gusmao said it would prepare to form a new government.
"It is set up to resolve the current political deadlock,” Gusmao said."These six political parties met to put hands together in order to go ahead and form a new government."