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 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" 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." It adds: "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".
SNP deputy leader at Westminster Stewart Hosie 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."
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."
Commenting on the report, a Scotland Office spokesman said: "The UK Government wants to see a single question in the referendum. This view is shared by the Scottish Government and all the main political parties in Scotland. It increasingly looks like a happy consensus in search of a problem.
"We respect the Scottish Government's right to ask a single, straight question on independence, as it promised. 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. The Secretary of State for Scotland has written to the First Minister to again offer a meeting to agree a legal, fair and decisive referendum."