THE greater resolve of Yes supporters to vote in September's referendum will give the independence campaign a clear advantage on polling day, according to new analysis.

The study has found the Yes ­supporters' determination to cast their ballot is likely to add two percentage points to the independence campaign's share of the final vote.

If the findings of the ScotCen social research centre are correct, the two sides appear to be effectively neck and neck, with 138 days to go to the referendum judging by one recent poll.

An ICM survey last month put support for Yes on 48% compared with 52% for No when don't-knows were discounted - showing the pro-independence campaign would require a swing of two percentage points to draw level.

However, a YouGov poll for Channel 4 News last night showed a much wider 16-point gap between the two sides, with support for Yes on 42% and for No on 58% after the undecideds were stripped out.

Better Together, the cross-party pro-UK campaign, last night said the ScotCen study showed No voters had to fight hard for victory.

The respected Edinburgh-based ­institute based its analysis on nearly 1500 responses to the most recent Scottish Social Attitudes Survey. Based on recent polls, it predicted an overall turnout of between 70% and 80%.

It found 90% of clear Yes supporters were more likely than not to cast their ballot on polling day, compared with 86% of clear No supporters, giving the pro-independence side a four-point advantage.

Looking at undecided voters, the study, conducted over a five-month period last year, found those leaning towards No were 19% less likely to say they would probably cast their vote than those leaning towards Yes.

Taking all voters into account, the report said the greater likelihood of Yes supporters voting "is enough to add two points to the Yes side's estimated share of the vote".

It described these as "differences that could matter".

Dr Jan Eichhorn, research fellow at Edinburgh University, who compiled the data, said: "While the overall turnout in the referendum is expected to be high, around 70% to 80%, there could still be a higher turnout among Yes than No supporters. In a tight race, this could be crucial."

Professor John Curtice, of Strathclyde University, the research consultant at ScotCen, said: "The proportion of those who say they are likely to vote has increased as the campaigns have developed.

"Far from putting people off, the campaigns are resonating with the public. But both campaigns evidently still need to make sure their supporters participate in the referendum come polling day."

A Better Together spokesman said: "This analysis shows that everyone who believes that we are stronger and better together as part of the UK has to campaign for it and they have to vote for it. This is the biggest decision that we will ever take as a nation.

"It is too important to leave to other people."

Yes Scotland chief executive Blair Jenkins said: "This suggests that more and more people are listening to the real facts and figures instead of 'Project Fear' scaremongering. They realise that, of the two futures facing their country, they are more likely to come out and vote for independence."

Figures from the National Records of Scotland showed 98,000 of 16 and 17-year-olds have been registered to vote in the referendum, representing about 80% of the total.

In all 4.12 million people - the country's biggest ever electorate - are signed up to take part in the historic vote.

Deputy First Minister Nicola Sturgeon said: "It is excellent news that so many of Scotland's young people have seized the opportunity to choose what type of country they want Scotland to be.

"Our young people have responsibility for Scotland's future so it is only right that they are able to have a say in the most important vote to be held in Scotland for three centuries. And this level of interest clearly demonstrates that giving the vote to 16 and 17-year-olds is clearly the right thing to do."

SNP business convener Derek Mackay said the Channel 4 News/YouGov poll was "encouraging", adding that it provided "further proof that more and more people are moving to Yes as the referendum draws closer".

"In recent months, support for Yes has grown significantly. A poll of polls published today found average support for Yes rising from 38% in November last year to 45% in April. Meanwhile, support for a No vote is falling steadily."