Johannesburg - The FF Plus urged President Jacob Zuma on Thursday not sign the transport laws and related matters amendment bill on e-tolling.
"The president is being asked to rather use his constitutional discretion to send the amendment bill back to Parliament," Freedom Front Plus transport spokesperson Anton Alberts said in a statement.
"We have established that the amendment bill has certain shortcomings and is unconstitutional."
He said the bill was incorrectly classified as a section 75 bill, which did not affect provinces, when it should have been classified as a section 76 bill, which affected all provinces.
"The Constitutional Court has, in the past, decided that a bill which has been classified incorrectly is void from the outset," Alberts said.
"The FF Plus believes that if the president does not send the amendment bill back to Parliament to correct the mistakes, a court application could be brought, which would declare it void."
The National Council of Provinces adopted the bill in May, despite the objections of opposition parties.