The MSP Brian Monteith has resigned from the Conservative group at Holyrood after being accused of disloyalty.
Brian Monteith has resigned after an internal party investigation
The move follows an internal probe into his conduct during David McLetchie's taxi expenses controversy and he could now face expulsion from the party.
Mr McLetchie resigned as Scottish Conservative leader on Monday.
Mr Monteith had e-mailed a Sunday newspaper editor suggesting a leader column calling for Mr McLetchie to quit as party leader.
Mr Monteith said: "I fully accept the seriousness of the matter and deeply regret the difficulties that my misjudgement will cause to my friends, colleagues and the party.
"Under the circumstances, it is therefore only right that I resign the Conservative whip with immediate effect."
The Acting Scottish Conservative leader Annabel Goldie told BBC Scotland that she was "appalled" at the circumstances surrounding the Mid Scotland and Fife MSP's resignation.
"The Tory group at Holyrood and the Conservative Party in Scotland had been not just dismayed but deeply concerned about press allegations that an MSP or MSPs had been briefing against David McLetchie," she said.
"The matter was the subject of a very robust investigation.
"What ultimately emerged today made it crystal clear that behaviour which was completely unacceptable to a member of the Conservative group had taken place and there was only one possible course of action to deal with that.
"We cannot and will not tolerate disloyalty of such a material nature. It's unacceptable."
Miss Goldie said that in recent weeks members of the Conservative group at Holyrood had "been expressing increasing frustration at the willingness of one or two colleagues to brief behind backs".
She said that following Mr Monteith's resignation from the group it would now be up to the Scottish Conservative chairman and the party organisation to determine whether he should be expelled.
Mr Monteith told reporters that he intended to stay on as a list MSP until the 2007 Holyrood elections.