Scottish residents have until midnight on Tuesday to submit their registration forms to vote in the independence referendum.
Scottish First Minister Alex Salmond has put out a call for the "missing million" who are not registered to vote to get out and harness "Scotland's immense wealth".
Labour leader Johann Lamont said it is the people least likely to vote who stand to lose the most if Alex Salmond "has got his sums wrong".
Yes Scotland leader Blair Jenkins said last minute voters could be crucial, insisting the referendum result "is on a knife edge".
But Better Together said "more and more Scots are saying no thanks" to independence.
Mr Salmond said: "David Cameron said this weekend that he is 'nervous' about the referendum result - and he is right to be so, with the most recent polls putting a Yes vote within touching distance, and hundreds of thousands of Labour voters set to back independence as momentum is now firmly with the Yes campaign.
"If you have friends or family who want to vote and are not registered, they have just hours to do so - this is their last chance to seize the wealth of opportunity offered by Scotland's historic referendum."
Ms Lamont said: "Everyone should take their opportunity to have their say as this decision could have a big impact on all our lives. It's a once in a lifetime opportunity and there will be no going back if we get it wrong.
"We all know that Scotland faces deep cuts if Alex Salmond gets his Yes vote and my fear is that many of those who don't vote will pay the price if he has got his sums wrong.
"The people least likely to vote are possibly those in most need of the good jobs that will be risked, the schools and hospitals that will be cut back and the state pensions which would be uncertain."
Mr Jenkins said: "If you are not registered, do it today because your vote could be crucial. it could decide whether Scotland's future is in Scotland's hands, or remains in the hands of the likes of George Osborne, David Cameron and Nigel Farage.
"The referendum result, and the wellbeing of services like the NHS, could hang on the decision that thousands of Scots make today about registering."
A Better Together spokesman said: "The choice we face is very clear. We can have what the majority of Scots want - more powers for Scotland without taking on all the risks of independence. It's the best of both worlds. Or we can take a leap into the unknown with separation.
"We don't know what currency we would use, how pensions would be paid for or how we would fund our schools and hospitals. That's why more and more Scots are saying no thanks to the risks and uncertainties of independence."