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ROBBIE DINWOODIE and MICHAEL SETTLE
Former Scottish Secretary Lord Forsyth of Drumlean said the issues had huge implications and should not be decided in "closed corner negotiations".
The landmark deal between David Cameron and First Minister Alex Salmond will be signed on Monday in Scotland after months of negotiations.
The referendum ballot paper is expected to have a single Yes/No question on whether Scotland should leave the United Kingdom.
It is also expected that the agreement will make provisions for those as young as 16 to vote in the referendum.
Lord Forsyth said: "Matters of electoral importance and the extension of the franchise are not matters to be carried out in closed corner negotiations, however senior the parties.
"If the franchise is to be extended for the referendum in Scotland, is it not inevitable that we will have to extend it to 16-year-olds in all directions throughout the United Kingdom – a matter that has huge implications, not least in that it will bring politics into our schools.
"If the Government is proposing to do that, would it not be proper for them to issue a paper for consultation, to consult widely and make no commitments whatsoever until they have done so."
Replying for the Government, Advocate General for Scotland Lord Wallace of Tankerness said the franchise for all elections to the UK Parliament and devolved parliaments had been set by Westminster and there were no plans to change it
The referendum voting age move was welcomed by the chairman of the Scottish Youth Parliament, who predicted the referendum will pave the way for 16 and 17-year-olds to be allowed to vote in all elections.
Grant Costello said that would make the referendum a "seminal moment for democracy".
He added: "Allowing 16-year-olds to vote in this referendum will pave the way for the franchise to be extended in all future elections.
"After all, if we trust young people to vote on Scotland's constitutional future, then we must surely see the case for allowing them to vote in all elections."
Meanwhile, the Electoral Reform Society (ERS) described as "surreal" the rush towards a single question, which it said ignores the wishes of the majority of Scots voters, condemning it as "a politicians' fix".
Willie Sullivan, Scottish director of the ERS, said: "It's surreal that politicians are refusing to discuss what many Scots want.
"Instead of talking about the Scotland we want to be, they are fighting over the two things that the majority simply don't want.
"It's politics that's disengaging from the people and not the other way round."
Mr Salmond, speaking in Edinburgh, insisted there is still work to be done to finalise the decision on the staging of the referendum.
He said Scottish Secretary Michael Moore and Deputy First Minister Nicola Sturgeon – leading negotiations for the two governments – still had to finalise a deal.
He said: "Great progress has been made and I'm certainly hoping and optimistic that a deal can be done next week. But a deal ain't done until it's finalised. There are a number of issues that are still to be resolved.
"I hope they can be resolved. That's the expectation."
The landmark signing of the deal between Mr Cameron and Mr Salmond on Monday will mark the final transition from the weeks of negotiations about the shape of the referendum to the public campaign.
The Herald, which exclusively revealed on Monday that a deal had finally been done between London and Edinburgh, has been told that the essential agreement had to all intents and purposes been sealed several weeks ago.
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