The deputy leader of the Progressive Unionist Party John Kyle has quit over "differing opinions".
The Belfast-based GP is one of the most high-profile elected representatives for the PUP.
He has served on Belfast City Council since 2007 when he was co-opted following the death of former party leader David Ervine.
It is understood he will now sit as an independent unionist.
In recent weeks, Mr Kyle had spoken out about the Northern Ireland Protocol, claiming it had potential to offer "significant opportunities" if fundamental changes were made to it.
Most unionists are fiercely opposed to the post-Brexit arrangements which include further checks on goods arriving into the region from Great Britain, regarding it as a border in the Irish Sea.
The PUP's policy is that the protocol should be scrapped.
In a statement on Thursday morning the PUP said Mr Kyle had tendered his resignation.
"Recent media appearances have shown how his personal opinions were differing from party policy that we had laid out, but we had hoped that John would stay within the party and carry on the work for his community as a member of the Progressive Unionist Party," it said.
"Unfortunately that is not the case.
"Dr Kyle has been a valued member of the party for many years. Undoubtedly this is not a situation we welcome but we do wish John well and thank him for all his endeavours on behalf of our party and his service to the community."
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