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 as a list MSP, is yet to comment.
A Reform spokesperson said: “Amanda Bland should reflect on her reasons for choosing to isolate herself from that team of capable individuals.
“It is incumbent on her to resign as a regional MSP for Central Scotland and Lothians West and allow her constituents to be represented by the next Reform candidate on that list.”
Under Holyrood’s rules, a list MSP would be replaced by the their party’s next candidate at the last election if they resigned.
Bland, a former primary school teacher and police officer, was among 17 Reform candidates to be returned to the Scottish Parliament in May as the party made its electoral breakthrough north of the border.
The party finished in joint second with Labour, behind the SNP on 58 seats.
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.