A multi-option referendum on Scottish independence, further devolution or the status quo would have "fatal defects", a committee of MPs has found.
The Commons Scottish Affairs Committee has accused the SNP of "political opportunism" by refusing to rule out a question on "devo max".
The committee is composed entirely of unionist MPs following the withdrawal of SNP MP Eilidh Whiteford over a dispute with its convenor, Labour MP Ian Davidson.
The fourth report from its inquiry into "the referendum on separation for Scotland" is published on Wednesday and focuses on the proposals for a multi-option referendum.
The report states: "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.
"Leaving aside the charges of political opportunism which can quite fairly be laid against the Scottish Government in pursuing this option, the evidence we heard shows very clearly the challenges and defects of the notion.
"The Scottish Government does not have a mandate to hold a referendum on greater devolution. What it promised was a referendum on separation."
"Devo max" is generally defined as full fiscal autonomy, giving Scotland full control over taxation and spending with defence and foreign policy reserved to Westminster.
However, the committee regards "devo max" as "no more than a phrase in search of content" and found "very serious unanswered questions about how a three-option referendum would work".
It stated: "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 counting mechanism could well determine the outcome. That is not acceptable.
"The outcome should be determined by the choices of voters — it should be clear to them what the consequences of their decision will be.
"It is probably for this reason that multi-option referendums are very uncommon internationally, on national issues of this sort.
"International experience strongly suggests that they are inappropriate because they do not lead to effective decisions. That would be true for Scotland."
Meanwhile, a separate report by the apolitical House of Commons Library released last week found that there are precedents for multi-option referendums internationally.
New Zealand held three referendums on electoral reform in 1992, 1993 and 2011, all with multiple options on a variety of voting methods.
Options where three questions are presented on one paper are rarer.
The library report stated: "One example is Puerto Rico, where voters were asked in 1967 and 1993 whether they wanted the island to be independent, to become part of the USA or remain as a Commonwealth linked to the USA.
"The Scottish Government has previously preferred two questions, with separate ballot papers: on support for devo max and on support for independence.
"A number of academic commentators have argued that either the alternative vote or the condorcet method would more reliably express the will of the people.
"Under the condorcet method, there would be three votes, for each option, status quo, devo max or independence."
SNP deputy leader at Westminster Stewart Hosie claimed Mr Davidson exposed the committee's inquiry as "bogus" last week when he accused the BBC of being biased in favour of the SNP.
He said: "The terms and timing of the referendum must be decided in Scotland, by the Scottish Parliament — not dictated by a Tory-led Westminster Government — and that includes a possible 'more powers' option, which is supported by a broad range of opinion in Scotland, including the STUC.
"It is only right that these matters are carefully and properly considered, which is exactly what the Scottish Government are doing."
He added: "The latest polls put support for independence ahead of both the status quo and more devolved powers, and we believe that we can and will achieve a Yes vote for independence in the autumn 2014 referendum."
The committee insists that the SNP has no mandate to hold a referendum on devolution as it won its landslide Holyrood election with a manifesto pledge to give Scots "a vote on full economic powers through an independence referendum".
This contrasts with the unionist parties, which won a greater share of the Scottish vote in the previous year's Westminster election with commitments for further devolution.
The coalition Government subsequently devolved more powers in the Scotland Act and Prime Minister David Cameron has since pledged to consider further devolution if Scotland votes No to independence.
With support for independence rarely rising much above a third in the polls, the committee has accused the SNP of trying to pull "an opportunistic political manoeuvre" to rescue "something from a prospective referendum defeat".
It states: "It is hard to avoid the conclusion that there are some in the SNP and the Scottish Government looking for schemes of further devolution to be added to the referendum on separation as an insurance policy against the verdict of the Scottish electorate."
On Monday, former prime minister Gordon Brown warned that "devo max" would lead to deeper cuts in public expenditure or higher taxes.
Former Liberal Democrat MSP Jeremy Purvis is leading a campaign calling for devolution of further tax powers short of full fiscal autonomy, known as the Devo Plus Group.
He said: "We welcome the recognition of the Scottish Affairs Committee that the only organisation at present developing detailed plans on further devolution is Devo Plus."
He added: "Our work in meeting with the parties, trades unions, business groups and others, to make our case, continues to be constructive and we agree with the recommendation of the committee that is for those political parties and organisations that support devolution to make proposals for developing it."
Labour's Shadow Scottish Secretary Margaret Curran said: "The committee's report highlights some of the significant difficulties about the practicality and regulation of a multi-option referendum where one of the options is undefined.
"At best, we would have confusion about the result the day after the poll and, at worst, change could be made without majority support amongst Scots."
A Scottish Government spokeswoman said: "The Scottish Government acknowledges the strong support within Scotland for a second question on more devolved powers.
"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."
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