The shock resignation comes on the day Reform hopes to cause an shock in the Hamilton, Larkhall and Stonehouse by-election.
John Swinney has repeatedly described the contest as a “two-horse race” between the SNP and Reform.
The word of the Hamilton, Larkhall and Stonehouse by-election: scunnered
Clashes with press, allegations of racism and misinformation - by-election highlights
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Mr Yusuf gave no formal reason for his departure.
He tweeted: “Eleven months ago I became chairman of Reform. I have worked full time as a volunteer to take the party from 14 to 30%, quadrupled its membership and delivered historic electoral results. I no longer believe working to get a Reform government elected is a good