The SNP’S majority at Holyrood has been cut to just one after John Wilson quit the party.
In a shock move, the MSP cited “increasing conflict” over the nationalists’ defence policy including its position on Trident and change of heart about membership of Nato.
The Central Scotland representative appeared to toy with the idea of following John Finnie and Jean Urquhart when they resigned the whip after a divisive debate over membership of the nuclear alliance at the party’s conference in Perth almost two years ago.
Mr Wilson said: “I’ve made a decision to go, to resign from the SNP and the SNP whip.
“It was in terms of policy about Nato. There was a divergence of policy differences.”
Mr Wilson, who was elected in 2007, was understood to be on the verge of being deselected for the Holyrood vote in 2016.
An SNP spokeswoman said: “We wish Mr Wilson well and accept his decision which, of course, comes at the same time as more than 26,000 people have joined the SNP in recent days, overtaking the Lib Dems UK-wide and over doubling our membership.
“The SNP continues to have a majority in the Scottish Parliament.”
She continued: “The SNP democratically decided our position on Nato at party conference two years ago, and we remain absolutely committed to ridding Scotland of Trident.”