Former Labour MSP Graeme Pearson has been appointed to lead the pro-UK campaign group Scotland In Union.
Pearson, a former police officer who was head of the Scottish Drugs Enforcement Agency, will be responsible for the day-to-day running of the organisation and will act as its official spokesman.
He takes over these roles from the group's founder, Alastair Cameron, at a time of mounting speculation about the prospect of a second Scottish independence referendum.
First Minister Nicola Sturgeon has told Theresa May her plan to take the UK out of the single market makes another vote on the issue more likely.
The debate over Scotland's constitutional future was reignited after the UK voted to leave the European Union, against the wishes of 62% of Scottish voters.
Pearson said: "Like most people, I hoped the decisive result in the 2014 referendum on Scottish independence would be the end of the division and uncertainty created by our constitutional debate.
"Regrettably, the nationalists have refused to take no for an answer, and are now using the Brexit decision as a new grievance, despite the fact Scotland clearly doesn't want another referendum.
"I believe that all of us who don't want another referendum should stand up and be counted.
"That is why I have decided to join the Scotland in Union campaign, and I would encourage everyone who agrees with me to make their voice heard by doing the same.
"Together, we can send a clear message that the nationalists do not speak for us and by removing the threat of another referendum, we can focus on achieving our ambitions for our country."
Mr Cameron, who will remain as executive director of Scotland in Union, said: "Graeme is a widely-respected figure, of great integrity and competence, who brings a wealth of experience from his time in the police and in politics.
"His appointment marks a new phase in our campaign to make a positive case for Scotland in the UK, and to oppose another referendum which would bring unwelcome and unnecessary division to Scotland."