Carla Lockhart has resigned from the Democratic Unionist Party (DUP) over its handling of the Drumcree parading dispute.
The Upper Bann MP, who is also an outspoken Christian, will now sit as an independent unionist at Westminster, following a growing disagreement with the party leadership over its response to the controversy.
She said it was a "painful" decision to make but was one made "after much reflection" and on a “point of principle”.
Lockhart had repeatedly called for First Minister Michelle O'Neill to be removed from office after she joined protests against an Orange Order parade in Portadown last month.
O'Neill and several other senior Sinn Féin figures have been interviewed by police over their involvement in the protest, which prevented the parade from travelling along the mainly nationalist Garvaghy Road.
The dispute intensified after Northern Ireland Secretary Sir Chris Bryant banned the parade on 9 October, following advice from the PSNI chief constable that it could not go ahead without an unacceptable risk of serious injury and disorder.
Lockhart criticised the DUP for failing to take stronger action against O'Neill, saying: "It cannot be business as usual at Stormont".
She also demanded that the party withdraw its Deputy First Minister, Emma Little-Pengelly, from office, which would automatically force O'Neill from her role.
However, DUP leader Gavin Robinson said Little-Pengelly would not take part in joint engagements with O'Neill, and that permission would not be granted for the First Minister to operate on a solo basis.
Lockhart had also criticised the party's handling of post-Brexit trade arrangements, particularly checks on goods entering Northern Ireland from Great Britain under the Irish Sea border deal.
The MP was among those critical of the agreement reached by the DUP in 2024 to restore power-sharing at Stormont, after the party had boycotted the institutions for around two years.
In her resignation letter, she cited concerns over the implementation of the Irish Sea border as another reason for leaving.
Lockhart was first elected to Westminster in 2019, having previously served as a councillor in Lurgan and an MLA for Upper Bann.
She retained her Westminster seat at the 2024 general election.
Responding to her departure, the DUP said the only winner from further divisions within unionism was Sinn Féin.