Angolan lawyers, who argue that President Jose Eduardo dos Santos' billionaire daughter was illegally appointed as the CEO of the state oil company, have applied for their case to be heard in Angola's Constitutional Court.

Angola's Supreme Court on Dec. 27 rejected an application by 12 human rights lawyers to have Isabel dos Santos removed as the head of Sonangol, which handles the oil and gas reserves of Angola, which is Africa's largest oil exporter.

The lawyers, who accuse the president of nepotism and violating Angolan probity law, asked the Supreme Court on Jan. 3 to allow the Constitutional Court to hear an appeal against its ruling.

"We have filed an extraordinary appeal at the Supreme Court, who transit the process to the Constitutional Court," said David Mendes, one of the attorneys.

"Nobody should get advantage based on their origin."

Mendes and the 11 other lawyers also argue that the seven months taken by the Supreme Court to come to a decision break guidelines on prompt rulings.

However, Jose Carlos, a lawyer not among the 12, thinks it will be difficult to prove any illegality by the president.

"The authors of the appeal would have to be clear and deep to prove the unconstitutionality of the appointment, which would be a surprise and very unlikely to be found," Carlos said.

Ranked as Africa's richest woman by Forbes magazine, Isabel dos Santos, 43, was given the job in June, prompting critics to accuse the president of trying to control state resources.

Jose Eduardo dos Santos, 74, has been in power since 1979 and is one of Africa's longest serving leaders. He says he will step down in 2018.

Watchdog group Transparency International ranked Angola 163 out of 168 countries in its index of perceived corruption in the public sector, and said it had "minimal" budget openness.

Isabel dos Santos says she was given the job because of her business acumen, and she has pledged to root out waste and corruption at a company that was struggling to stay afloat even before oil prices plunged.

She holds controlling stakes in several companies in Angola and in Portugal, its former colonial ruler, including energy, mobile telecoms and banking sector investments.