The option of greater devolution for Scotland - so-called "Devomax" - should be included somewhere on the ballot paper in the looming Scottish independence referendum, political scientists and pollsters have told MPs at Westminster.

The Scottish Affairs committee was taking evidence for a report on the vote, which will happen before the end of 2014.

Peter Kellner, president of the polling company YouGov, told MPs there was clearly significant support for Devomax and it needed to be tested at the polls in some way.

"It is quite clear that Devomax is a major player and has popularity," he told MPs, saying it would be "perverse" to rule it out.

But constitutional experts agreed that exactly what was on offer in terms of greater devolution needed to be a lot more clear, and that the Scottish and UK governments needed to clarify this urgently.

"We know that people say that they want more, but you could say they've been offered a menu without prices," said Professor Iain McLean, Fellow in Politics at Nuffield College, Oxford.

He said that while there was "no one optimal solution" in how to set the questions in the referendum, there was a "danger of a binary referendum taking place when the underlying public opinion is not binary".

Negotiations between London and Edinburgh are continuing, but there is little sign of an agreement in the offing about what should be on the referendum ballot paper.

David Cameron has insisted that the referendum, whenever it comes, should be a straight yes/no decision, at least in the first instance.

Alex Salmond is said to prefer there being three questions - with a Devomax option coming after the in-or-out options. This has been rejected by the coalition in London, which believes it could create confusion if the outcome saw a narrow majority for independence, but an even larger one for Devomax.

Other experts agreed that there was a strong case for the question of Devomax being put to the public at the same time as the more general question of a yes/no independence question.

Professor John Curtice from Strathclyde University told MPs that the impasse between the UK coalition government and the SNP could be solved by having two separate questions on the ballot paper.

"First of all have the straight, clear question, which is, 'Do you want Scotland to become an independent country or not," he said.

"Then you can have a second question on the paper, saying if Scotland remains part of the UK, do you want Devomax.'

"The truth is there is no one single option," he concluded, suggesting that most polls indicated the Scottish electorate was currently split three ways - with about a third of the population each wanting independence, the status quo or Devomax.

Professor Vernon Bogdanor from Kings College London said there was a "powerful argument" for holding a referendum on Devomax at a later date, assuming a straight yes/no vote produced a clear decision among Scots to remain in the UK.

But he agreed that Westminster would have to be clear - before the first referendum - what powers it was proposing to transfer. "There must be a clear specification, there can't be a mystery prize," he told MPs.

On the issue of which powers would be reserved to Westminster under a Devmoax solution, John Curtice said about two thirds of Scots wanted to see full control in Edinburgh over taxation and welfare payments. But he said that Scots were much less keen overall on Holyrood taking control of defence of foreign affairs.

"There is a clear dividing line," he said.