MSP Amanda Bland has quit Reform UK Scotland to sit as an independent.
The resignation relegates the party to the third-largest group at Holyrood, with one member fewer than Labour.
A Reform UK source told BBC Scotland News that Bland was "very difficult to work with" and had been called to a meeting on Wednesday to discuss a high turnover of staff in her office.
Bland, who represents Central Scotland and Lothians West, said her "principles and priorities remain unchanged" but gave no details of why she had left the party.
In a statement she said: "I entered politics to champion fairness and common sense, protect women and children, improve public services, support businesses to thrive, and work towards a fairer and more effective criminal justice system.
"As a member of the criminal justice committee, I remain committed to scrutinising legislation, holding the justice system to account and supporting practical measures that improve public safety, protect victims and ensure justice is served.
"Holding the Scottish government to account remains a key priority, and I believe I can best pursue these goals as an Independent MSP."
She said she would not be making any further statements or giving interviews about Reform, and that her focus now was on her responsibilities as an independent MSP.
Reform's cohort, led by former Conservative minister Malcolm Offord, have repeatedly invoked the condemnation of the chamber - most notably after staging a walkout during an independence debate, and by tabling a motion proposing a burka ban.
Currently, Offord is first to question John Swinney at First Minister's Questions every other week, alternating with Labour. The party is now likely to lose that privilege.
Scottish Labour deputy leader Jackie Baillie said: "No one will be surprised to see chaos at the heart of Reform."
She said the party had been "utterly useless at holding the SNP to account or getting results for the Scottish people", adding: "It's been a series of stunts, gimmicks, photo calls and precious little else."
The Scottish Conservatives said there were growing questions about Reform's ability to provide effective opposition to the SNP at Holyrood.
A spokesperson said: "With Nigel Farage's Reform being weak and ineffective at Holyrood, the Scottish Conservatives will continue standing up to the SNP and fighting for Scotland's taxpayers."
Scottish Liberal Democrat David Green MSP said that Reform had been hit by "scandal after scandal".
He added: "I'm sure more and more of its MSPs will be taking a long hard look at what they've signed up for."
The Scottish Parliament confirmed Bland's resignation and said its website, external had been updated accordingly.
Extracted and lightly reformatted for readability. · Source: pt