Reform Scotland Justice Spokeswoman Amanda Bland has quit the party to stand as an independent, leaving Scottish Labour as the largest opposition party at Holyrood.
Bland did not disclose the specific reasons for her decision to Reform, but said the move came after "careful deliberation".
She went on: “My priorities and principles remain unchanged. 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.”
Bland's resignation leaves Malcolm Offord's Reform Scotland with only 16 MSPs and means that, for the first time in a decade, Scottish Labour is now the single second biggest party in Holyrood.
The parties had previously both been tied with 17 seats, ahead of the Scottish Green Party with 15 and the Scottish Conservatives with 12. The SNP has 58 seats.
A Reform statement, The Times reports, called for Bland to step down as an MSP. It said: “We are proud of a talented and united team of MSPs at Holyrood who have supported each other at every step of the way. 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 Central Scotland and Lothians West and allow her constituents to be represented by the next Reform candidate on that list.”