The historic deal to be signed at St Andrew's House today by Prime Minister David Cameron and First Minister Alex Salmond will hand Holyrood the right to hold a legally binding referendum, including establishing the franchise.
This had been described as a poisoned chalice as Scottish ministers would be left with a UK-controlled electoral register containing only some 44,000 "attainers" – young Scots aged almost 17 who would attain the age of 18 in the following year, rather than as many again who would reach the age of 16 in time for the referendum.
There have been predictions this could leave the poll open to a legal challenge, but a Scottish Government source told The Herald last night: "A mechanism has been found to ensure the franchise will go beyond the attainers. It will allow the Scottish Parliament to fix this."
On the eve of the signing and handshake between the two Government heads there was disagreement about which side had struck the better bargain.
Tory peer Lord Forsyth lambasted Mr Cameron for betraying the anti-independence cause in negotiations, but another former Scottish Secretary, Sir Malcolm Rifkind, insisted a "comprehensive defeat" had been inflicted on Mr Salmond by blocking the option of a second question.
Current Scottish Secretary Michael Moore also rejected Lord Forsyth's attack, saying he regretted the intemperate language comparing the Prime Minister to Pontius Pilate and lambasting his "appalling negotiating skills".
He added: "People will see what we have achieved together as what both Governments wanted – fair, legal, decisive and made in Scotland, a good agreement that now allows the big issues to be debated."
Deputy First Minister Nicola Sturgeon, who concluded final negotiations with Mr Moore on Friday, accepted that a possible second question had been conceded, but said: "Earlier this year David Cameron wanted to dictate the timing of the referendum, the wording of the question and the franchise. Now all of these will be made in Scotland."
On the franchise issue, a Scottish Government spokesman said: "The principle of lowering the voting age enjoys wide political support. The Liberal Democrat and Green Parties have expressed support for lowering the voting age for elections, and Labour supported a move to allow 16 and 17-year-olds to vote in the 2011 UK referendum on the Alternative Vote.
"Our consultation sought views on extending the franchise to 16 and 17-year-olds and subject to agreement between the First Minister and the Prime Minister on Monday it will be for the Scottish Parliament to decide how to take that forward."
Mr Cameron will today describe the deal as paving the way for a "people's referendum", saying: "Scotland's two Governments have come together to deliver a referendum which will be legal, fair and decisive. This marks the beginning of an important chapter in Scotland's story and allows the real debate to begin.
"It paves the way so the biggest question of all can be settled: a separate Scotland or a United Kingdom? I will be making a very positive argument for our United Kingdom.
"It is now up to the people of Scotland to make that historic decision. The very future of Scotland depends on their verdict. It is that important. This agreement delivers the people's referendum."
Mr Salmond said last night: "The people of Scotland gave the Scottish Government
a clear mandate in last year's election to hold a referendum on Scotland's future in 2014.
"The agreement I expect to reach with the Prime Minister is one which ensures that not only is the referendum made in Scotland, but that the fundamental right of the people of Scotland to choose their own future is respected by all.
"The agreement will see Scotland take an important step toward independence, and the means to create a fairer and more prosperous Scotland. I look forward to working positively for a 'yes' vote in 2014."
However, Martin Sime of the Scottish Council for Voluntary Organisations, which wanted the possibility of a second question on further devolution kept open, expressed disappointment at what he called a "politicians' fix".