DAVID Cameron has privately conceded the pro-Union parties will need to give voters more details on the alternative to independence ahead of the 2014 referendum, according to sources close to the meeting between the Prime Minister and First Minister last week.

Cameron is understood to have made the admission to Alex Salmond shortly after saying in a speech in Edinburgh on Thursday that he would consider giving more powers to Holyrood if there was a no vote in the referendum.

The Prime Minister's offer of "jam tomorrow" led to a flurry of questions from the media about which powers he had in mind, but Cameron conspicuously failed to identify anything specific, prompting scepticism from the SNP.

As Cameron's speech explicitly referred to the benefits of a UK-wide tax and welfare system, it is highly unlikely he would consider significant economic powers or so-called "devolution max".

After meeting him in the wake of the speech, the First Minister likened Cameron's offer to that of former Tory Prime Minister Alex Douglas-Home before the 1979 devolution referendum, who also pledged more powers in return for a no vote. Over the next 18 years, successive Conservative Governments failed to deliver on the deal.

Now it is understood that during the meeting at St Andrew's House, Cameron said it was incumbent on the No campaign to make clear what the alternative was to independence, effectively admitting there is a lack of detail at present.

The remark prompted Salmond to tell the Prime Minister to get on with it.

The Sunday Herald also understands Cameron appeared vague on recent aspects of the debate on devolution, and had to turn to Scottish Secretary Michael Moore for help when Salmond raised the SNP's six demands for amending the Scotland Bill currently going through the House of Lords.

Moore then repeated the Coalition mantra that the six demands, including control of the Crown Estate Commssion, corporation tax, broadcasting and excise duty, were still being considered.

Salmond yesterday stepped up the pressure on Cameron by calling on him to "set out the terms" of his offer on further devolution before the Scottish Government's own consultation on the referendum ends in May.

The First Minister's spokesman said: "David Cameron himself has now said that more powers for Scotland, beyond the current Scotland Bill, are 'on the table'. If he has this card up his sleeve, he has a democratic obligation to put it on the table – and to place it face up, so that the people of Scotland can see what it amounts to.

"Not only does this apparent offer need to be in the public domain well before the referendum, David Cameron should spell it out within the timescale of the Scottish Government's consultation on the referendum, which ends on May 11. That is plenty of time, if the Prime Minister and his colleagues are serious about it.

"The issue right now is the people's right to know what the alternative to independence is – now that David Cameron has said, for the very first time, that it is not the current Scotland Bill, he has an obligation in the coming weeks and months to tell the rest of us what it is."

In his Edinburgh speech, Cameron said he wanted Scotland to remain in the Union and make the most of the new powers in the Scotland Bill, but added that didn't have to be "the end of the road".

He said: "When the referendum on independence is over, I am open to looking at how the devolved settlement can be improved further. And yes, that means considering what further powers could be devolved.

"But that must be a question for after the referendum, when Scotland has made its choice about the fundamental question of independence."

Cameron's problem is that he wants the referendum kept on a narrow, constitutional footing, asking whether people want to be in or out of the Union.

However, the SNP have redefined the whole debate in more practical terms, by asking people want they want Holyrood to do rather than to be, and in particular what economic powers it should wield.

A Scotland Office spokesman last night said the SNP Government seemed "desperate to talk about anything other than what independence will mean for Scotland. The Prime Minister made clear the independence referendum needs to be resolved first.

"We should get on with that as soon as possible and determine whether or not Scotland is to remain part of the UK.

"The largest transfer of financial powers ever to the Scottish Parliament will begin shortly if the Scotland Bill is passed into law."

A Downing Street spokesman said: "The Prime Minister was clear in his speech, and in his meeting with the First Minister, that any discussions on further devolution should take place after the referendum, which should have a single question and take place as soon as possible."

Meanwhile, Scottish Conservative leader Ruth Davidson has asked Salmond to explain how he would pay for benefits and pensions if Scotland was to become independent.

Davidson said that in the most recent year for which figures are available, the amount spent on welfare in Scotland was almost three times more than oil and gas revenues.