Secretary Michael Moore said in an interview on BBC Prime Minister David Cameron and First Minister Alex Salmond will sign the agreement, which grants Scotland's Parliament the authority to hold the referendum sometime before the end of 2014.
"The challenge that has been placed to both governments in the last few months is to ensure we get a referendum which is legal, fair, decisive and is made in Scotland," Moore said. "I think when people see the details of what we announce tomorrow they will see we have achieved that."
Britain's The Guardian said the breakthrough came when Salmond accepted the idea of having a single question about breaking away from the United Kingdom on the ballot. Salmond had been pushing for inclusion of a second question on a political arrangement that would stop just short of complete Scottish independence from Britain.