In a significant blow to First Minister Alex Salmond, an Edinburgh University study found 60% of 14 to 17-year-olds were opposed to independence.
Just 21% were in favour, while 19% were undecided.
The findings – which also revealed teenagers were heavily engaged in the debate and highly likely to vote – emerged from a detailed survey of more than 1000 young people from across Scotland.
The SNP's campaign director, Angus Robertson, said previous polls had indicated higher levels of support for independence among young people.
He added: "The Edinburgh University survey shows that Scotland's students want more information about independence.
"In the coming months the Scottish Government will continue to publish a series of papers covering the main arguments for independence, leading to a white paper in the autumn that will set out detailed proposals for an independent Scotland.
"We believe that by September 18, 2014, most people of all ages will agree with the positive case for an independent Scotland."
A spokesman for the pro-independence Yes Scotland campaign said: "It's great to hear the views of these young people and Yes Scotland will be working very hard over the next 15 months to persuade them and others that Scotland's future should be in their hands, with a Yes in the referendum."
The pro-UK Better Together campaign welcomed the findings.
Michaella Drummond, 17, a Better Together youth representative from Kirkcaldy, Fife, said: "This poll may surprise a lot of people, but it doesn't surprise me.
"I know that the overwhelming majority of my friends and the majority of people my age that I speak to simply cannot see the sense in Scotland going it alone.
"I cannot comprehend nationalism. What the SNP is proposing seems out of touch with modern society."
Scottish Labour's constitution spokeswoman Patricia Ferguson said: "It is refreshing to see young people showing the confidence to say No, despite the assumptions made about them."
The survey also showed 94% of youngsters were aware of the referendum and only 11% had not discussed it with friends, family or classmates.
Two-thirds of the teenagers questioned said they would like more information before they finally make up their minds.
However, those planning to vote No appeared more confident of their view, with 39% saying they had enough information to make a decision compared with 32% of those backing independence.
Girls were less likely to support independence than boys, reflecting the gender gap revealed in opinion polls. The survey showed 17.3% of girls planned to vote Yes, compared with 25% of boys.
The low overall level of support for independence surprised researchers.
The SNP's determination to give 16 and 17-year-olds a vote was taken as a sign the party believed teenagers would be more likely to back a split with the UK.
However, support proved lower than among the wider electorate, where backing for a Yes vote has ranged from 28% to 36% in polls this year.
Dr Jan Eichhorn, one of the report's authors, said: "I think there is something about not knowing what will happen. A lot of young people don't feel they have enough good information to help them decide."
Holyrood is in the process of passing legislation lowering the voting age from 18 to 16 for the referendum.
Those aged 14-and-a-half when the Edinburgh University survey was conducted will be eligible to take part when the poll is held on September 18 next year.
In negotiations with the UK Government paving the way for the referendum, Mr Salmond fought hard for 16 and 17-year-olds to be allowed to vote.
The UK Government agreed, in what was seen as a concession. The Conservatives continue to oppose the move, though it is backed by the other main parties.
The votes of 16 and 17-year-olds are unlikely to prove decisive, according to polling experts.
An extra 120,000 people in the age group will be entitled to vote, or about three per cent of Scotland's four million-strong electorate.
The Edinburgh University survey, funded by the Economic and Social Research Council, quizzed 1018 teenagers in April and May.