MSP Michelle Ballantyne has resigned from the Scottish Conservatives and will see out the Holyrood term as an independent.
The South Scotland MSP left the party nine months after she was defeated in a leadership contest by Jackson Carlaw.
She cited policy differences with new leader Douglas Ross - who replaced Mr Carlaw - saying she and the party were no longer "a good fit".
Mr Ross thanks Ms Ballantyne for her service and wished her well.
The former Scottish Borders councillor entered Holyrood in May 2017 via the South Scotland list after fellow Tory Rachael Hamilton quit to successfully contest a by-election.
She served as the party's social security spokeswoman, but was dropped from the frontbench team by Mr Carlaw after she contested the leadership election against him.
Mr Carlaw had been the clear favourite for the post - and won comfortably - but Ms Ballantyne said the party should not have a "coronation".
However when Mr Carlaw himself quit the leadership six months later, she stood aside as Mr Ross took the role without a contest.
Ms Ballantyne broke the party whip to vote against the latest set of coronavirus restrictions on Thursday.
She told the Telegraph newspaper that travel restrictions being enforced in law had been "the straw that broke the camel's back", adding that the party "need to question a lot harder" the government's approach.
In a statement confirming she was resigning the whip, the MSP cited "differences" on policy with the new leadership, saying: "Sadly for me this means I no longer feel the party and I are a good fit."
In a joint statement with group leader Ruth Davidson, Mr Ross thanked Ms Ballantyne for "her service to the Scottish Conservatives over the years".
The resignation leaves the Scottish Conservatives with 30 MSPs until the election next May.