South Africa's two biggest mobile operators, MTN and Vodacom, have joined number three Telkom in a lawsuit to stop the regulator withdrawing temporary frequency spectrum assigned at the start of the COVID-19 outbreak.

The temporary spectrum allowed the operators to deliver faster connectivity to customers to meet a surge in data demand as large numbers of people shifted last year to working and studying from home.

MTN and Vodacom have also used the temporary frequencies to launch 5G networks.

But the Independent Communications Authority of South Africa (Icasa) said in August telecom operators must return their bandwidth by the end of November, as it could not allow the temporary assignment to "assume a state of permanence," while a permanent process was underway.

That permanent process - of auctioning long-term spectrum - was delayed by legal wrangling, after Telkom and broadcaster eMedia Holdings argued the auction process was flawed and obtained a court order to suspend it.

After that legal wrangling was resolved, Icasa restarted the auction process this month.

Telkom then filed new papers this month asking the High Court to set aside Icasa's decision to withdraw temporary spectrum, saying the removal of that spectrum would have a "catastrophic" impact on its network performance and customers.

"Vodacom can confirm that it has joined the legal proceedings and that it will be supporting the relief sought by Telkom," Vodacom said in a statement on Monday.

MTN said it had filed papers in the Pretoria High Court on Friday opposing the decision by Icasa that requires the return of the temporary spectrum.

Since the start of the pandemic, the demand for data on MTN's network alone, has increased by 165%, MTN said.

"The withdrawal of the temporary spectrum will also pose a significant risk to lower income South Africans, students and learners who are benefiting from free access," established at the start of the pandemic, the operator said.

More than five million people get free access to more than 1 000 websites, thanks to the temporary spectrum, MTN said, adding that it remains open to settling out of court.

Last week Icasa said it would challenge Telkom's lawsuit.