The new policy emerged in a letter from Mike Russell, the Constitution Minister, to SNP backbencher Aileen Campbell.

He said it was the Scottish Government’s intention to introduce the change “to the extent that this is practicable” and noted how matters of voting franchise were reserved to Westminster, so the issue would have to be specifically addressed in the forthcoming referendum legislation.

“It is my intention to make provisions to this effect in the Referendum Bill that essentially reflect the approach taken for the elections to NHS Boards in Scotland,” said Mr Russell.

The First Minister defended the policy, saying: “If people are able to marry and fight and pay taxes, then why on Earth should 16 and 17-year-olds not have a say in the future constitution of their country? You might argue that 16 or 17-year-olds have more invested in the future of the country than anybody else and therefore to deprive them of the franchise would be quite wrong.

“It is a perfectly legitimate argument to say that in a referendum on the future those who are going to be part of the future should have the right to vote.” Asked whether it was political opportunism, Mr Salmond replied: “That might be the case if we were opposed to extending the franchise to 16 for other elections but the SNP are in favour of extending the franchise for the Scottish elections, for the Westminster elections, for the Scottish health board elections, which we’re currently trialling through the Scottish Parliament at the present moment.”

However, Pauline McNeill, Scottish Labour’s spokeswoman on the constitution, branded the SNP’s proposal “desperate”.

She said: “The fact that the SNP are proposing it for their referendum, which would account for an additional 3% of those who voted, we believe is a cynical attempt by the SNP to keep this referendum issue going.

“We, once again, call on the SNP to get on with the real business of government.”

A poll in 2007 suggested the policy of independence is particularly popular among younger people with 47% of 18 to 24-year-olds backing the idea. Sir Tom Hunter, the business tycoon, criticised the timing of holding a referendum during an economic downturn. He argued the focus should be on economic renewal rather than the constitution.

In a newspaper article, he wrote: “We have to get our priorities right; debating constitutional reform when our economy is teetering on the edge is like rearranging the deck chairs on the Titanic. We have to address the urgency of our economy first.”

Sir Tom made clear that it was absolutely legitimate to have a referendum but it was wrong to divert significant energy to such an important issue as political reform when the focus should be on economic renewal.

But the First Minister defended his plan, saying that the recession reinforced the need for Scotland to have more autonomy.

“We’re not proposing to have a referendum now, we’re proposing it next year when we believe the economy will be moving out of recession, and we also believe that the arguments for having economic powers are actually being made by the situation we are in at the present moment.”

Mr Salmond added that the fundamental point he took from Sir Tom’s views was that he “actually believes there should be a referendum on the constitution so that seems to me the really substantial point, and, of course, in that he is in agreement with the vast majority of the people of Scotland”.