Isabel dos Santos, daughter of Angola's former president and Africa's onetime richest woman, must return to Angola her shares in Portugal's Galp energy firm worth 422 million euros ($500 million), an international arbitration court has ruled.

Dos Santos is accused of diverting billions of dollars from state companies during her father Jose Eduardo dos Santos's nearly 40-year rule of the oil-rich southern African nation.

The embattled ex-first daughter, whose business assets have been frozen since 2019, was ordered by a Dutch court this week to return shares worth $500 million to Angola's national Sonangol energy group, which she chaired until Lourenco took power.

The transaction under which Dos Santos acquired her stake in the oil and gas company Galp is "null and void", according to a copy of the ruling seen by AFP on Friday by the Netherlands Arbitration Institute (NAI), which is part of the International Court of Arbitration.

After paying a 15 percent deposit from the bank account of another company in the British Virgin Islands, dos Santos allegedly paid the rest of the amount in Angola's local currency, worth little outside the country, rather than in euros as agreed on the sales contract, according to the NAI.

Santos's six-percent stake in Galp is part of a myriad of investments in Angola and former colonial ruler Portugal, worth about $3 billion according to Forbes magazine, that have been under scrutiny.

The court's decision -- dated July 23 and first reported by Dutch media late Thursday -- said that the 2006 purchase of the shares, acquired through a company owned by dos Santos' late husband Exem Energy, was illegal.

Dos Santos had consistently denied any wrongdoing and denounced all accusations as a politically motivated witch hunt.

Exem's lawyers intend to appeal the decision "with the competent court".

"In this arbitral award the political narrative clearly overrides the legal analysis," the company said in a statement emailed to AFP on Friday.

One of Sonangol's lawyers, Yas Banifatemi, told Dutch media there was "nothing political" in the court's decision.

Isabel Dos Santos is accused of diverting billions of dollars from state companies during her father's rule of Angola
Isabel Dos Santos is accused of diverting billions of dollars from state companies during her father's rule of Angola AFP / MIGUEL RIOPA

"The arbitration court has judged that Isabel dos Santos enriched herself with money stolen from Angolan people," said Banifatemi, cited in Dutch daily newspaper Het Financieele Dagblad.

President Joao Lourenco has vowed to crack down on corruption since dos Santos retired in 2017, removing his predecessor's cronies from key positions and probing the former regime for alleged graft.

He has targeted several members of the dos Santos family, including Isabel and her younger brother Jose Filomeno dos Santos, sentenced to five years in prison for diverting oil revenues last year.

Isabel is the eldest daughter of Angola's ex-president, accused of ruling the country with an iron fist, leaving a legacy of poverty and nepotism.

The British-educated billionaire businesswoman has faced several allegations of plundering the public purse and funnelling the money abroad.

In a trove of 715,000 files released in January 2020 by the award-winning New York-based International Consortium of Investigative Journalists (ICIJ) and dubbed the "Luanda Leaks," dos Santos was accused of syphoning state funds from the oil-rich, but impoverished country into offshore assets.

Nicknamed "the princess" in Angola, she was accused of amassing her vast fortune thanks to the backing of her authoritarian father.

In Portugal, in addition to Galp, she has major bank stakes and has a controlling share of a Portuguese cable TV and telecom firm.

In December 2019, Angola's prosecutors froze the bank accounts and assets owned by her and her Congolese husband Sindika Dokolo, who died last year, a move she described as a groundless political vendetta.

Dos Santos became Africa's richest woman after Forbes magazine named her the continent's first female billionaire in 2013. She lost that title when her assets were frozen.