SUPPORT for independence has stalled as the SNP and Yes campaign enter a critical year for the debate on Scotland's future.

A new poll puts public backing for a split at 28% – unchanged from last October. It means the pro-UK campaign has a 20-point lead, more than double the nine-point advantage it enjoyed this time last year.

However, the Yes camp will take comfort from a recent fall in opposition to independence, which is down from an all-time high of 53% in October to 48%. Over the same period the number of don't-knows has risen from 19% to 24%, the highest level since the TNS BRMB series of polls began in 2007.

The softening of opposition follows the signing of the Edinburgh Agreement – due to be rubber-stamped by MPs later this week – which will hand Holyrood legal powers to stage the 2014 referendum.

For the first time in a major poll, the views of 16 and 17-year-olds were included. But they appeared not to alter the overall picture despite hopes in the Yes camp they would edge opinion towards independence.

The findings came as First Minister Alex Salmond pledged to outline what he called the "why" of independence in the coming year after months of wrangling over technical issues. But the latest poll shows how much ground he and the Yes campaign have to make up.

Since October new monthly polls, published together for the first time today, have shown little movement.

Support for independence dipped to 27% in November – an all-time low in the TNS BRMB series – and stood at 28% in December and this month. Opposition slipped from 50% in November to 48% in December, where it has remained.

The polls have coincided with a difficult period for the Scottish Government, which has faced criticism over its claims about an independent Scotland's EU membership and its plans to enter a currency union with the UK, and suggests support for independence is at or near a bedrock level. UK Government warnings about defence jobs were also in the headlines as this month's poll of more than 1000 voters in 55 constituencies was taken.

Chris Eynon, head of TNS BMRB Scotland, said: "The main effect of the Edinburgh Agreement between Cameron and Salmond in October was an increase in the undecideds and a slight decline in opposition to independence from its peak of 53%, rather than any more positive influence on opinion.

"Similarly, the recent debate around automatic EU membership or not following independence has had no adverse effect on levels of support, which might suggest these current levels of opposition and support are based much more on principle than the detail. They are a reflection of the heart rather than the head at this stage."

He added: "While this gives the Yes campaign a sizeable base on which to build and a significant 24% undecideds to attempt to win over, the gap of 20% remains large. If the opposition is consolidating at around 48%, the challenge is considerable.

"It is worth noting the introduction of those aged 16 to 17 to the sample has no effect on overall opinion, such is the low level of their representation within the electorate as a whole."

Mr Salmond yesterday said: "We will

explain how, by extending and completing the powers of the Scottish Parliament, we can deliver a fairer society for all the people of Scotland."

He said Westminster's backing for tough welfare cuts would prove a "key moment" in the independence debate, insisting an independent Scotland could "do better".

His comments were seized on by Scottish Labour's constitution spokeswoman Patricia Ferguson, who said: "It is an astonishing admission of failure from Alex Salmond that after 80 years of the SNP, the best part of 20 years as leader and more than five years as First Minister, he is only now getting round to explaining why Scotland should be independent."