• Sri Lanka is facing its worst economic crisis since independence in 1948.
• The island nation of 22 million declared last Tuesday that it was going to default on its billions of dollars of foreign debt for the first time in its history.
• In a cabinet reshuffle on Monday, President Gotabaya Rajapaksa removed two of his brothers and a nephew from the new team.

Sri Lanka's opposition is set to stage a no-confidence vote against the government, opposition lawmaker Harsha de Silva told CNBC on Monday.

"We are confident we have the numbers and we will bring the motion at the appropriate time," said de Silva, a parliamentary member from the opposition Samagi Jana Balawegaya party.

His comments came before President Gotabaya Rajapaksa appointed a new cabinet on Monday. Following pressure from protesters who are calling for his resignation, the president removed two of his brothers and a nephew from the new team, but retained another brother Mahinda Rajapaksa, who is the prime minister.

Widespread protests with chants of "Gotta go Gotabaya" against the president and the prime minister have dragged on for the past month as Sri Lankans struggle with 12-hour power cuts and an extreme scarcity of food, medicines and fuel.

"We will bring it when the time is right because it is our no-confidence motion and it is our prerogative when we bring it," de Silva told CNBC during an interview on "Capital Connection Asia." He refused to specify if the no-confidence vote against the prime minister will be tabled on Tuesday, when the parliament convenes.

Sri Lanka is facing its worst economic crisis since independence in 1948. The island nation of 22 million declared last Tuesday that it was going to default on its $35 billion of foreign debt for the first time in its history.

The country will be sending a delegation, including its finance minister, to hold a meeting with the International Monetary Fund this week to seek a bailout.

The opposition has also demanded that the president repeal a two-year-old constitutional amendment that vests his office with extraordinary powers, de Silva said.

According to the 20th amendment, the president cannot be removed — he can only step down or be impeached. It was introduced after Rajapaksa took office, a move observers say has eroded democratic features of Sri Lanka.

The 20th amendment rolled back most of the reforms in the 19th amendment, which imposed certain limits to the president's authority and removed his blanket immunity from legal proceedings.

De Silva said an offer to repeal the 20th amendment could be a form of compromise to resolve a weeks-long political impasse.

"A compromise move at this time for the president is to say he is willing to repeal the 20th amendment and go back to the much more democratic 19th amendment," de Silva said.

The opposition MP said there were few viable political options left.

"Impeachment is a long-drawn-out process. It needs a two-thirds majority and the Supreme Court has to agree. It will take time," de Silva said, noting that the president had the power to remove the prime minister and his cabinet. 

De Silva said it was not yet clear if some ruling party dissidents would vote with the opposition on the no-confidence motion.

He said there were 42 MPs who expressed a willingness to sit in opposition benches. "If they do, the government will immediately lose the majority," he added.