Carla Lockhart has quit the Democratic Unionist Party (DUP) in a row over its response to the Drumcree parading dispute.
The MP for Upper Bann had been calling for the party to withdraw Stormont’s deputy first minister to force First Minister Michelle O’Neill from their joint office.
Lockhart said she was leaving the DUP on a “point of principle” and would continue in the House of Commons as an independent unionist.
The fall-out comes after unionist anger over Sinn Féin’s involvement in protests on Garvaghy Road, which blocked a contentious Orange Order parade from passing along the mainly nationalist street in Portadown, County Armagh.
O’Neill and several other senior Sinn Féin figures have been interviewed by police investigating the demonstrations, with unionists accusing the party of acting unlawfully.
Last week the DUP, which shares power with Sinn Féin in Northern Ireland’s devolved government, announced it would block official engagements involving O’Neill in response to the fall-out.
The Sinn Féin vice-president has defended her involvement in the protests, saying it was the “right thing to do”.
DUP leader Gavin Robinson stopped short of Lockhart’s call to force O’Neill from office.
In an open letter to colleagues last week, Lockhart said it was the “only one credible option” and any kind of “graduated response” would be “insulting to those who have trusted us”.
In a statement posted on her Facebook page on Monday, the MP said: “My first responsibility remains to the people of Upper Bann.
“You placed your trust in me, and I will continue to work every bit as hard for you, your family and our community.”
Lockhart said it was a “sad day and a deeply personal decision” but her departure from the DUP was “on a point of principle”.
“I cannot stand idly by whilst Sinn Féin wilfully ignores the rule of law and the promised profound consequences fail to materialise,” she added.
Lockhart concluded that she would “look forward to working with all shades of unionism, elected or not, to advance the cause we love”.
In a statement, the DUP said: “We regret Carla Lockhart’s decision to resign from the Democratic Unionist Party. We thank her for her service to the party and recognise her work on behalf of the people of Upper Bann.
“The only winner out of further divisions within unionism is Sinn Féin.”
It added: “We share the anger felt across unionism at Michelle O’Neill’s actions. Our disagreement is over the course of action that will secure accountability and best serve Northern Ireland.”