National Assembly Speaker Baleka Mbete has rejected a request to vote by secret ballot during the motion of no confidence against President Jacob Zuma.
On Thursday, UDM leader Bantu Holomisa threatened to go to court if Mbete failed to respond or refused the opposition's request.
He says there have been threats of intimidation deterring ANC MPs from voting against Zuma.
The motion of no confidence debate has been set down for a special sitting of Parliament on 18 April.
Holomisa says opposition parties are duty-bound to seek another legal remedy.