The Scottish Labour, Conservative and Liberal Democrat leaders will today issue an unprecedented joint statement committing their parties to giving Scotland more powers, including over tax, in the event of a No vote in September.
But they will stop short of pledging an exact programme, in part because of policy disagreements between the parties.
The statement comes barely a fortnight after the Conservatives unveiled the findings of their devolution commission at the start of this month.
A pro-Union source said: "There is not going to be a line-by-line agreement [from the three parties] and there should not be a line-by-line agreement. Because there are areas where we do disagree.
"The purpose of this is not to tell people this is exactly what you are going to get. That would be inappropriate. New powers have always been brought about by consensus. Not just among political parties but in consultation with civic society too."
He said that today's event would be about "showing people that more powers are going to be delivered".
But he denied that the main purpose of the statement was to counter the SNP's claims that there are no guarantees Scotland will get any extra powers after the independence referendum. All three pro-Union parties have set out different visions of what extra powers should be given to Holyrood, including over the ability to raise tax and on welfare.
But all three parties are in agreement on a number of policies that they oppose, including the SNP's call for corporation tax to be devolved to Scotland.
Their "statement of intent" comes after two polls at the weekend gave a boost to the Yes campaign. An ICM poll of 1003 people found support for independence at 36 per cent, two percentage points up on an equivalent poll last month.
Meanwhile, a Panelbase survey commissioned by the pro-independence Yes Scotland campaign showed that support for a Yes had reached a "campaign high" of 48 per cent, once 'don't knows' were removed.
But amid growing fears over the tone of the debate, the ICM poll also found almost two in five people in Scotland believe that the country will be left "badly divided" whatever the outcome of the referendum.
Pro-independence politicians also set out their visions for Scotland today.
Deputy First Minister Nicola Sturgeon will say that an obligation to work towards nuclear disarmament should be a legal requirement in an independent Scotland and included in the country's interim constitution.
A draft of the document, due to be published shortly, will set out arrangements such as the role of government and the protection of human rights in the early days after a Yes vote.
Separately, First Minister Alex Salmond will also outline new powers for Scotland's islands on a two-day visit to Orkney.
These are expected to include measures that will give local communities more control over their economies and thier environments.
Ahead of the launch in Kirkwall, Mr Salmond said that he hoped that the prospectus he will offer would be "seized across all of our islands".
South of the border, Scottish Secretary Alistair Carmichael also backed the idea of devolution for England. He said the current constitutional settlement was an "anomaly", which allowed him to vote on matters that did not affect his constituents.
The Lib Dem cabinet minister said he personally favoured federalism as a potential solution to the long-running issue.
Both camps also saw more high-profile campaigners give them their backing.
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