The UK government has poured cold water on hopes that votes for 16-year-olds in the Scottish referendum on independence could be extended to the rest of the UK.
Plans to give the vote to 16- and 17-year-olds in a referendum on Scottish independence came under fire in the House of Lords on Wednesday, and the UK government has said the measure will not necessarily lead to them voting in all UK elections.
First Minister Alex Salmond will meet Prime Minister David Cameron on Monday to finalise a deal over how the referendum will be staged.
That is likely to see an independence referendum held north of the border in the autumn of 2014 - the Scottish Government's preferred timescale.
It is expected that voters in Scotland will be given a straight choice between independence or remaining in the United Kingdom. It is also expected that 16 and 17-year-olds will be allowed to take part in the crucial ballot.
Leader of the Scottish Youth Parliament Grant Costello said: "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."
Former Scotland secretary Lord Forsyth of Drumelan said the issues had "huge implications" and should not be decided in "closed corner negotiations".
Tory Lord Jopling said it was "a major constitutional change" and Labour's Lord Foulkes of Cumnock questioned how extending the electoral register to include younger votes would be funded.
But asking an urgent question in the Lords, 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 and 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 franchise for referendums is set out in the legislation that enables each referendum to take place," he said.
"If we agree to transfer powers to the Scottish Parliament to hold a referendum then it is they who would determine the franchise."
He said any decision to allow 16- and 17-year-olds to vote in the referendum "would not affect the franchise for Parliamentary elections".
Lord Jopling, a former government chief whip in the Commons, asked Lord Wallace: "Surely you will accept that the introduction of a reduction in the voting age to 16 and 17 would be a major constitutional change and that normally major constitutional changes are produced and proposed only after clear consultation and very often with a Speaker's conference.
"In the view of very many people, to produce this like a rabbit out of the hat is quite unacceptable."
The Scottish government has already extended the franchise to 16 and 17-year-olds for elections to health boards.