The Fiji Labour Party leader, Mahendra Chaudhry, says any change to the 1997 constitution by referendum would be illegal.
Mr Chaudhry was responding to a statement by the attorney general, Qoroniasi Bale, that the government is considering changing the constitution by introducing a Referendum Act.
He has told Radio Fiji there is no other way to change the constitution apart from the procedures laid down in the constitution itself.
He says a referendum is not part of the constitution.
"The only manner in which the constitution can be amended, added to or changed in any way is through the procedures outlined in the constitution itself. And as the constitution stands, a referendum is not part of that process. A bill to amend the constitution has to be presented to the House and that bill has to be passed and it requires a majority of two thirds of the House to put through a constitution amendment bill that is the legal way of amending the constitution. Anything done outside that constitution is unconstitutional, extra-legal nd it will not be lawful."