The indy undecideds: Yes camp need to persuade 75%, poll finds

Mark Diffley, research director of Ipsos MORI Scotland, said the contest was far from a foregone conclusion given that only 56% of Scots say they are certain to vote and already know which side they will support.

But he said the Yes camp faced a "considerable challenge" to win over enough don't-knows to cancel out, or overtake, the pro-UK side's commanding two-to-one lead among voters whose minds are made up.

Ipsos MORI's polling data suggests pro-independence majorities are concentrated in deprived neighbourhoods around the Central Belt and Aberdeen, where voters are more open to change.

Mr Diffley said: "The trick in this (for the Yes campaign) is concentrating on that group, finding out more about them, finding out what floats their boat.

"People are more willing to make change because they have less at stake and less to lose.

"All the things people are worried about apply to people in these areas less than in affluent areas.

"The tone of the Yes campaign is that there is not going to be massively radical change.

"But given that where the support lies seems to be where people are in favour of more radical change, is that the right strategy to have?

"That's the question that arises from the data."

He added: "You could argue the offering could be more radical because it  might pick up more people in those areas."

He said a more radical vision of independence was unlikely to deter those already committed to voting Yes but warned a switch in strategy could invite attacks from the pro-UK side.

He added: "I would not suggest this is an easy solution for the Yes campaign."

The Yes campaign need to capture about seven out of 10 of the 44% of Scots who are  undecided.

The group is made up of those who are not certain to vote (the biggest single section of don't-knows) and those who will vote but either do not know which side to support or admit they might change their mind before polling day.

Mr Diffley said there was no evidence to suggest the group as a whole differed significantly from those whose minds are already made up.

In another indication of the uphill struggle faced by the Yes campaign, he said more pro-independence supporters were in the group who admitted they might change their mind.

However, with many Scots turned off by the present "noisy and shouty" debate, he said the Scottish Government's keenly awaited white paper on independence - the blueprint for a go-it-alone Scotland expected in November - or polls next year predicting a Tory victory in the 2015 Westminster election could help sway opinion towards a Yes vote.

Up to now the SNP-dominated pro-independence campaign has emphasised continuity over change.

First Minister Alex Salmond has said the Queen would remain head of state and claims an independent Scotland would keep the pound as part of a currency union with the rest of the UK.

The Scottish Government would also seek to share the UK's welfare system, in the short term at least.

The First Minister's aides have privately described the approach as "indy lite".

Questions over the strategy were also raised by the SNP's former leader, Gordon Wilson, who said Scots should choose whether to retain the monarchy and keep the pound in fresh referendums if they vote for independence.

He urged the Yes campaign to launch an unashamedly negative assault on the "cancer" of London and Southern England.

Spokesman for Yes Scotland, the cross-party pro-independence campaign, said: "As Mark Diffley, the research director of Ipsos Mori Scotland pointed out, there is everything to play for in this referendum.

"We have been working hard and - will continue to work hard - to provide people with the quality information they need to see that a Yes vote is the best way to create a fairer and more prosperous country.'

A spokesman for the pro-UK Better Together group said: "There is all to play for in the referendum and anyone who thinks that either campaign has this sewn up is playing a very dangerous game."