A Lisbon court has seized a 26 percent stake in Portuguese telecommunications firm NOS from a holding company that is co-owned by Isabel dos Santos, the daughter of the former Angola president and reputedly Africa's richest woman.

Dos Santos, who was named a suspect in a fraud investigation in Angola this year, owns 50 percent of the holding firm ZOPT, Portugal's first four play operator with products which include mobile phone services, broadband and a TV cable operator broadcasting Warner Bros and Disney releases. 

Telecommunications company Sonaecom, a sub-holding of Portugal's Sonae, owns the other 50 percent of ZOPT, and announced the court seizure of shares worth €411 million ($444.1 million) on Saturday night.

Read more: Isabel dos Santos to sue reporters' consortium behind 'Luanda Leaks'

While the seizure affects only half of ZOPT's shares, equivalent to dos Santos' holdings - ZOPT as a whole will be deprived of voting rights.

Sonaecom has argued that the decision to do so "seriously harms the interests of ZOPT and Sonaecom, both being third parties completely unrelated to the judicial process underway.”

Sonaecom said it would contest the decision as ZOPT "is not liable for the debts of its shareholders.”

Read more: Isabel dos Santos slams Angolan court for seizing $1 billion

Angolan courts froze the assets of the daughter of former President Jose Eduardo dos Santos in December, alongside those of her husband's after it was alleged that the two took part in deals with state-owned companies that ended in government losses of $1.14 billion.

Sonaecom has pointed out that when the assets were frozen in December, voting rights were not frozen.

In February, Portugal's public prosecutor also ordered the seizure of bank accounts belonging to the ex-first daughter, who also owns shares in several other Portuguese companies including oil group Galp.

Watch: In The Spotlight: Isabel dos Santos, Angola's 'princess' and her troubled business empire

Sonaecom said it would cooperate with judicial authorities while taking "all appropriate legal action to revert and terminate the seizure procedure."

Earlier this year, hundreds of thousands of files about dos Santos, dubbed the Luanda Leaks, were released by several news organisations focusing on how she amassed a fortune estimated at more than $2 billion.