David Cameron has dubbed one of the SNP's flagship policies FFS - "Full Fiscal Shambles" - as he accused the party of being obsessed with process not policies.
The Conservative leader's nod to a more colourful phrase came after SNP Westminster leader Angus Robertson accused ministers of failing to deliver on their extra devolution promises.
As the Herald reported earlier this week, MPs were briefed by the House of Commons Library that potentially "controversial" elements of the Smith agreement are not included in the Scotland Bill currently going through Westminster.
In response to Mr Robertson, the Prime Minister accused the SNP of dodging questions about which extra powers in the Bill they would use.
Mr Cameron said the truth was that the "SNP only want to talk about process.
"They don't actually dare talk about which power they are being given they would like to use.
"You don't like the way things are fixed, why don't you put up taxes and spend more money? ...
"Because the truth is this - full fiscal autonomy has now become FFS - Full Fiscal Shambles."
He added that Full Fiscal Autonomy would "land Scottish taxpayers with a bill of thousands and thousands of pounds."
And he challenged the nationalists: "If that's your policy... you should say so."
The SNP dispute the figures, and point out that the UK already runs a budget surplus.
Mr Robertson had asked at Prime Minister's Questions: "The Prime Minister and other UK party leaders made a promise, they made a vow that more powers will be delivered to the Scottish Parliament....Why does your Scotland Bill not even deliver the limited Smith Commission proposals?"
Amendments tabled by the SNP indicate the party will try to resurrect FFA when MPs debate the Scotland Bill again next Monday.
Ian Murray, Labour's shadow Scottish Secretary said: "We know that the SNP's plans for full fiscal autonomy would blow a £7.6 billion black hole in Scotland's finances.
"The SNP have refused to spell out the cost of full fiscal autonomy and refuse to say when it would be introduced. It is a total mess and economically illiterate."
Labour will try to amend the Bill to order an analysis of the impact of FFA, which the party says will give Scots "the facts" on the policy.
Labour also plans to table a series of amendments on welfare.
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