Ballymena DUP councillor Davy Tweed is leaving the party over the prospect of it going into government with Sinn Fein.
Ian Paisley and Gerry Adams face dissension within their parties
Mr Tweed said such a move by the DUP would "not so much be a backward step as a complete somersault".
Meanwhile, Fermanagh Sinn Fein councillor Poilin Ui Cathain is quitting her seat over Sinn Fein's decision to support the police.
She said she hoped her reservations about the move would be proven wrong.
Mr Tweed, a former Ulster and Ireland rugby player, said he sent a letter of resignation to the party on Tuesday but had yet to receive a reply.
He said he felt betrayed by the DUP.
He said the party was "falsifying its intentions" and would be "far better coming out and being open and transparent with people".
Mr Tweed said the DUP had attacked former Ulster Unionist leader David Trimble over the Good Friday Agreement, but in his view the St Andrews Agreement was worse.
He said as more people got to know the details of St Andrews, they were becoming opposed to the DUP's stance.
Mr Tweed will stay on Ballymena Borough Council as an independent.
Sinn Fein's Ms Ui Cathain, a councillor for Erne West, said her decision to step down from the council was prompted by Sinn Fein's backing for the PSNI at a special party conference in Dublin last month.
She has not left the party.
She told the Fermanagh Herald newspaper she was unable to support the party's new policy on policing.
"I sincerely hope that my reservations regarding Sinn Fein's support for the Police Service of Northern Ireland will be proven wrong," she said.
"I believe that this decision is now irreversible. Therefore while I can afford to be wrong, I don't believe that Sinn Fein has that luxury."
A Sinn Fein spokesperson said it was comfortable with the councillor's decision to resign her seat, saying it respected her point of view.