The head of the Better Together campaign has denied plans to set out a timetable for giving more powers to Scotland were a sign of panic.
Alistair Darling was speaking after a poll suggested the pro-UK campaign had lost its lead.
The SNP has dismissed a promise by the Chancellor to unveil a timetable for further devolution if voters reject independence as a last-minute “bribe”.
George Osborne said on Sunday that a “plan of action” would be set out in the next few days to give “more powers to Scotland; more tax powers, more spending powers, more powers over the welfare state”.
BBC Scotland’s political correspondent Glenn Campbell has reported that a new body will also be set up to hammer out more powers for Holyrood if there is a “No” vote on 18 September.Speaking to BBC Radio 4’s Today programme, Mr Darling acknowledged that the referendum race was “clearly very tight” but said there was no panic.
He added: “We are in a position now where every voter in Scotland could potentially tip the balance in the referendum.
“But I am confident we will win because we do have a very strong, positive vision of what Scotland can be, both in terms of the opportunities and the security that come from being part of the UK, a strengthened Scottish Parliament, with more powers which is what people want and you can do that without having to break up the country to do it.”
Mr Darling stressed that no new powers would be put on the table beyond those already announced by the three main Westminster parties earlier this year.
He added: “The additional powers coming to the Scottish parliament were announced by the party leaders, north and south of the border, some time ago.
“People have said, ‘Yes we want to know the timetable and the process’ and that is something the government is going to announce this week.
“But remember this, this is a referendum on whether or not we stay on the United Kingdom. It is not a referendum on what further powers we are going to get. We are going to get them anyway, if we stay in the United Kingdom.”
Labour leader Ed Miliband has joined Mr Osborne in saying the process of handing more powers to Scotland should begin immediately after any “No” vote.
SNP leader Alex Salmond said it was a “panicky measure” announced without credibility because his Yes Scotland campaign was “winning on the ground”.
Neither Prime Minister David Cameron nor any other Conservative Cabinet Minister is expected to visit Scotland this week, but Downing Street denied any suggestion of complacency over the referendum result.
Finalise the details
A spokesman for Number Ten was also unable to give any details of when an announcement would be made on setting up a new body to agree a timetable for handing more powers to Scotland. He said this would be an issue to be settled by the political parties.
It is understood discussions are still going on between the three main parties to finalise the details.
Number Ten also confirmed that no contingency plans are being put in place for a possible “Yes” vote in the referendum.
Scottish Finance Secretary John Swinney told the BBC that it was a fair assessment of the polls to say the Yes campaign remained behind in general, but said his experience was that undecided voters were moving to Yes by a factor of two to one.
He added: “The movement is in our favour and the campaign is exciting an enormous amount of interest and activity and participation on the ground across Scotland. I think it is the formidable strength of the ‘Yes’ campaign that is motivating that.”
Referring to Mr Osborne’s announcement, Mr Swinney said: “There is nothing new being offered this week. We may well get a timetable but the substance, the actual powers, the things that matter, Alistair Darling made absolutely crystal clear yesterday in contradicting George Osborne, that on the substance there will be absolutely nothing new.
“If I look at the different offering of the the Labour party, the Liberals and the Conservatives they are all different. I can’t answer to you today, neither could Alistair Darling, what would be the proposition that people get under this alternative scenario? So it is vague and it has all been offered before.
“The second point is that in 1979 Scotland was told vote ‘No’ in the referendum and you’ll get a stronger parliament and what we got was a Conservative government for 18 years that we never voted for, industrial devastation and no parliament.
“So I think the moral of the story of recent history in Scotland is if you want a guarantee of strong powers for the Scottish Parliament you have to vote ‘Yes’ in the referendum a week on Thursday.”
Canon Kenyon Wright, who chaired of the Scottish Constitutional Convention that paved the way for the creation of the devolved Scottish Parliament in 1997, said Mr Osborne’s announcement had a “whiff of desperation about it”.
Canon Wright, who now supports independence, added: “It’s now clear that devolution has two main problems. One is that it is incomplete and will never cover the areas needed to protect the people of Scotland on issues like welfare. The second is that it is insecure and always will be, so long as sovereignty remains at Westminster.”
BBC political correspondent Ben Wright said the outcome of the referendum now looked “utterly uncertain”.
He said the “sleepy assumption” in Westminster among the pro-Union parties that there would be a relatively comfortable vote against independence had been completely destroyed over the past few days.
Voters in Scotland go to the polls on Thursday 18 September, when they will be asked the “Yes/No” question: “Should Scotland be an independent country?”
In the last full week of campaigning, Mr Miliband is expected to be joined on the stump by Gordon Brown.
The former Labour prime minister has said Westminster must deliver on its promise of further devolution so “a No vote doesn’t mean nothing happens”.