Lady Sylvia Hermon has said the Ulster Unionists' decision to oppose the devolution of policing and justice had been a mistake.
The UUP sole MP said Tuesday's 'no' vote went against all she thought the party stood for.
The North Down MP also described the attack on Northern Ireland secretary Shaun Woodward over the issue as "bewildering".
"The real enemies are not fellow unionists in the DUP. Those are not our enemies. In fact Sinn Fein is not our enemy," she said in a BBC interview on Friday.
"The enemies in this community are dissident republicans who, if they had their way, would continue to wreak havoc across Northern Ireland.
"I wanted all of the parties in the executive to stand shoulder to shoulder in the face of dissident terrorism and say, 'We have had enough. We are going forward together.'"
Last month, Lady Hermon dealt a blow to the UUP's electoral alliance with the Conservative Party when she refused to stand on the joint ticket.
The wife of late Northern Ireland chief constable Jack Hermon has been an opponent of the Tory link-up since it was agreed in 2008.
She has supported Prime Minister Gordon Brown on a number of key votes from the opposition benches at Westminster and has refused to take the Tory Whip.
She is understood to be "weighing up her options" and speculation is mounting that she may stand as an independent.
When asked if she should follow her critics' advice and resign from the party, she said: "Perhaps they are right."
The UUP has branded her remarks as "regrettable".
"The Ulster Unionist Assembly Group took its decision to oppose the devolution of policing and justice at this time with the full support of the Ulster Unionist Party Officer team and with the full support of the Party Executive Committee", a UUP statement said.
"This is how the UUP make democratic decisions."
"It is regrettable that Lady Sylvia's position does not reflect the view of the party and it is disappointing that she failed to voice her concern at the Party Executive meeting rather than in the media."
The party added there was "widespread anger" following Shaun Woodward's attempt to "pressurise and bully the UUP".
The Ulster Unionists and the Conservatives are fielding joint candidates in Northern Ireland's 18 constituencies for the first time.
© UTV News