The letter, written by the pro-UK campaign chief Alistair Darling, calls for a "fair referee and fair rules" for the 2014 vote. It was sent to the First Minister yesterday after 27,385 people put their names to it.
The Scottish Government wants to put the question: "Do you agree that Scotland should be an independent country?" on the ballot paper.
The wording has been criticised by a number of polling experts and psychologists who claimed it is loaded because it invites an affirmative answer.
The impartial Electoral Commission is reportedly about to reject the Government's phraseology.
Better Together's letter follows repeated warnings by ministers and SNP MSPs that they might over-rule the Electoral Commission – whose role is only advisory.
Blair McDougall, the Better Together, campaign director, said: "If we are to have a fair referendum, then there needs to be someone completely independent of both campaigns setting the rules."
A Scottish Government spokesman said: "It is for the Government to propose the question, for the Electoral Commission to test those proposals and it will ultimately be for the Scottish Parliament to decide on the question."