Scottish independence: 'No mandate' for multi-referendum
The Scottish government has no mandate to hold an independence referendum with a second question on increased Holyrood powers, a Commons report has said.
Westminster's Scottish affairs committee also warned a multi-option referendum had "fatal defects".
Scots ministers have said their preference is for a single question on independence in the 2014 referendum.
But political opponents claim the SNP wants a second one as an insurance policy, in case of a "No" vote.
The Scottish government said it "acknowledges the strong support" for a multi-option ballot paper.
The Scottish Affairs Committee is currently comprised entirely of MPs from the three major pro-union parties - the Conservatives, Labour and Liberal Democrats.
In its report, the cross-party committee said the SNP won the last Holyrood election on a commitment to a referendum on independence, rather than asking people if they wanted increased powers for the Edinburgh parliament as part of the union.
The committee report said, since devolution in 1999, opinion poll support for independence remained at "broadly the same level" and had "seldom exceeded one third of those responding to polls".
"It is perhaps unsurprising that a nationalist government looks for a way out, and tries to find a way of rescuing something from a prospective referendum defeat," it concluded.
"This alone would be enough to condemn the the idea of a second question as an opportunistic political manoeuvre.
"The political authority that was conferred on the Scottish government by the Scottish election results was not the authority to call for a referendum on further devolution - it was for a referendum on separation, and that is the mandate which it has."
The report also said the result of a multi-question referendum could be open to interpretation, if more than one option won popular support.
Giving an example, public law professor Adam Tomkins, of Glasgow University, asked what would be the outcome of a referendum with a 65% turnout, in which 35% voted for the status quo, 40% backed full fiscal autonomy and full independence was supported by 25%.
The committee report concluded: "Widening the number of options to be put in front of the voters in a referendum may, at first sight, be an attractive proposition, but it suffers from a number of fatal defects.
"There are a number of potential ways in which the results could be calculated and aggregated and it is deeply disturbing to discover that the choice of voting and the counting mechanism could well determine the outcome. That is not acceptable."
A Scottish government spokeswoman said its consultation on Scotland's future got almost 10 times as many responses as a similar exercise by UK ministers.
"The Scottish government acknowledges the strong support within Scotland for a second question on more devolved powers," said the spokeswoman.
"The responses to our referendum consultation, which included asking for views on whether there should be a second question, are currently being independently analysed and this analysis will be published by the end of the summer."
The UK government, which wants a single-question independence referendum, previously saw the passing of the Scotland Act, which devolves new financial and other powers to the Scottish Parliament.
A Scotland Office spokesman said: "This is the most important decision Scotland has to make.
"It cannot be allowed to become confused with other possible forms of devolution which are an entirely different matter from Scotland leaving the UK."