LEN McCluskey has hinted that he personally would support Scottish independence.
The Unite union, of which he is the head, has a policy of "positive neutrality" on Scotland's future.
But Mr McCluskey has previously suggested that Alex Salmond's arguments for independence are "seductive".
Asked if he had a view about what he would do if he himself had a vote in the referendum, Mr McCluskey was cagey. "I do," he said, "and I'm going to keep that to myself.
"I'm not even going to give you the slightest hint, I'm trying desperately for none of my eyes to flicker."
Asked if he would say how he personally would have voted after the result is known, he replied "yes".
But he added: "People are not interested in my personal view, they are interested in my view because of Unite, and it would be an abuse of my position [to say]."
He added it was "regrettable" that Unite's call for a devo max option to be added the ballot paper was not listened to.
Mr McCluskey has previously said the SNP presented a challenge to Labour leader Ed Miliband, who needed to show he was "on the side of ordinary Scottish workers" to counter the appeal of Mr Salmond's party.
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