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.”

In a statement, Traditional Unionist Voice leader Jim Allister congratulated Lockhart on her decision.

“Having myself resigned from the DUP almost 20 years ago, I know this was not an easy decision. But for honest politicians principle matters more than position.

“The DUP’s preference for ‘O’Neill Must Stay’ caused them to choose Michelle O’Neill over Carla Lockhart. Little wonder their electoral base is deserting them.”

“Allister added that he looks forward “to working closely with Carla” and said it is “a hopeful day for unionism”.

BBC News NI political editor Enda McClafferty

Cutting links with a party she first joined as a teenager was a big call for Carla Lockhart.

Her family ties to the DUP run deep prompting one colleague to say, “Carla bleeds the DUP”.

But not anymore.

She has been on a collision course with the DUP leadership for weeks and now the “profound consequences” are being felt by the party.

Though she had some support within the DUP, she became a lone voice in her battle with the leadership.

She had the backing of Portadown Orangemen and many unionists, but Gavin Robinson crucially had the full support of the DUP officer and assembly teams.

He was able to face down the demands from his Upper Bann MP safe in the knowledge he had the numbers on his side.

Collapsing the executive without a budget agreed and running the risk of his party being blamed for a nurse’s pay dispute strike was not a gamble he was willing to take.

But it has come at a cost for Robinson as he has now lost an MP from the DUP benches leaving just four for a party which once boasted ten members of parliament.

It has been a bruising time and one he needs to recover from quickly with just seven months until the assembly election.

The party has lost seats in the last two elections and now its losing seats in between elections.