Support for keeping Scotland in the UK remains ahead in the independence referendum campaign, a new poll suggests.

A No vote is backed by 57% compared with 32% for independence and 11% who are still to make up their mind, according to the Ipsos Mori poll, commissioned by STV News.

When the "don't knows" are excluded, the result would be 64-36 in favour of the union.

The poll of 1,001 people was taken after Chancellor George Osborne said he would not accept a formal deal to share sterling with an independent Scotland.

Asked if the currency debate has shifted opinion, 34% of undecided voters said the intervention makes them more likely to vote No, compared with 16% who felt more inclined to vote Yes. Almost half said it made no impact.

But support for the union remains unchanged at 57% compared with the same poll in December. And overall, 39% of undecided voters are inclined to vote Yes compared with 29% leaning towards a No vote, STV reported. Support for independence is two points down from December.

Mark Diffley, director at Ipsos Mori Scotland, told STV: "Our latest poll for STV shows that public attitudes have changed very little over the past three months and that the No campaign appears to retain a healthy lead.

"The important recent debate over currency appears not to have shifted committed voters from their positions while undecided voters appear a little more inclined to vote No as a result.

"Having said that, overall support for a No vote has not changed since December. We are now into the final 200 days before the referendum takes place and both sides will be stepping up their efforts to win over those who are yet to decide how they vote.

"Time will tell whether this has a significant impact on public attitudes."

The poll results are based on a phone survey of people aged over 16 between February 20 and 25.

Better Together leader Alistair Darling said: "This poll makes clear that Alex Salmond's failure to tell us what would replace the pound if we left the UK is costing the nationalists support.

"The idea that voters can be expected to go to the polls blind on the most fundamental issue affecting our economy is simply not credible. What the people of Scotland need to hear from Alex Salmond is his Plan B. Would we rush to adopt the euro or would we set up an unproven separate currency?

"The only poll that counts is the one on the day. Although we speak for the majority of Scots, there can be absolutely no complacency. We will be doing everything between now and polling day to convince those who have yet to make up their mind that we are stronger and better together."

Blair Jenkins, chief executive of Yes Scotland, said: "Looking at all the polls since last November, average support for Yes is currently up three points at 41% while No is down three points on 59% - the research clearly shows there is everything to play for.

"And by far the most telling result from this poll is that undecided voters are more inclined to move to Yes, with nearly four in 10 more attracted to us compared to fewer than three in 10 inclined to No. This confirms our own research, and what our thousands of volunteers are reporting, that when people learn about the gains of Yes the more likely they are to support an independent Scotland.

"The No campaign - Project Fear - has played what it clearly believed was its ace card when Chancellor George Osborne said he would not share the pound with an independent Scotland, but it has not had the impact the No campaign would have been hoping for.

"So, following the No campaign's recent barrage of negativity and scaremongering, we take heart from the fact that Yes support is above 40% on an average of all the polls, and we will continue, with renewed vigour, to make the case that Scotland's future in Scotland's hands makes absolute sense."