The SNP have been accused of reneging on a key election pledge as Whitehall insiders insisted the party's primary aim of ensuring Holyrood gains full powers over tax and spending will not happen in this parliament.

The issue of Full Fiscal Autonomy (FFA) was thrown to the fore after SNP MP Tommy Sheppard admitted seeking its implementation quickly and scrapping the Barnett Formula would be "a disaster" for Scotland.

But UK Government insiders stressed the SNP's desire for full tax and spending powers had no chance of being realised before 2020 - and that there would be no negotiations.

One senior source, when asked if FFA was possible before the next General Election, said: "We are not looking at anything beyond the current Scotland Bill," while another declared: "This is the SNP's one opportunity(for change)."

As MPs debated the Scotland Bill that will give the Scottish Parliament more powers in wake of the Smith Commission recommendations, Willie Rennie, the Scottish Liberal Democrat leader, seized on Mr Sheppard's remarks, saying the Edinburgh East MP had "let the cat out of the bag".

"The SNP repeatedly disputed the assessment of the Institute of Fiscal Studies(IFS) that FFA would cost Scottish public services £7.6bn. Now a senior MP seems to agree," he said.

John Swinney, in London for talks with Chancellor George Osborne, when asked if FFA could happen before or after 2020, pointed out such a major change would take years to implement and stressed there was a "parliamentary process and intergovernmental discussions to be gone through".

Asked if, therefore, beyond 2020 for FFA was a reasonable timescale, he replied: "The timescale will be a product of the negotiation and the parliamentary process that is undertaken. We have acknowledged in our manifesto change of that nature takes time."

Stewart Hosie, the SNP's deputy leader, asked about FFA, suggested a staggered timescale with the Smith recommendations first, followed by more powers over welfare, business taxes and minimum wage and, "then as quickly as we can, given proper negotiations have to happen, we want the rest of those powers".

The Dundee MP insisted the party would be tabling a specific amendment to "deliver maximum powers" when the Scotland Bill reaches its detailed committee stage before the summer recess.

During a trip to America, First Minister Nicola Sturgeon was adamant that Scotland could "shoulder" the burden of cutting fiscal ties with the UK, telling US TV viewers that "of course" Holyrood could afford FFA.

During the Bill's debate, David Mundell told MPs there was simply no case for FFA in light of the IFS's projection that it would leave Scotland with a £7bn black hole in its finances, growing to £10bn by the end of the decade.

Introducing the Bill, the Scottish Secretary insisted it delivered in full the Commission agreement, signed by all the Holyrood parties following the independence referendum.

Insisting it would produce a "very stable settlement", Mr Mundell claimed it would redress the political balance by giving Holyrood not only responsibility for spending much of its money but also responsibility for raising it too.

"For Holyrood to be a powerhouse parliament that it rightly aspires to be - and that this Government wants it to be - it must be accountable to the people of Scotland for raising more of the money it spends. The Bill before us today is about ensuring that that missing link is fixed," declared the Secretary of State.

The SNP tabled an amendment arguing the measures outlined in the Bill were not an "adequate response" to the election result in Scotland, where it won 56 of the 59 seats, or the fulfilment of the Smith Commission agreement.

But it was not accepted for debate by Speaker John Bercow yet this did not prevent regular interruptions from the SNP benches throughout Mr Mundell's remarks.

Within minutes, Alex Salmond, the former First Minister, was making clear that the Bill failed to meet the standard set by Smith.

Mr Hosie called the Bill "so inadequate" that it did not even allow the Scottish Parliament to raise all of the money it spent.

However, Labour's Ian Murray claimed the Bill would give Holyrood the "immense power to change society for the better" to create a fairer Scotland.

The Shadow Scottish Secretary claimed Scots deserved an answer over the SNP's "fundamental broken promise" on FFA, saying the Nationalists had become "so uncomfortable with the policy" because they had realised its financial consequences would be severe for Scotland.

But Angus Robertson, the SNP leader, hit back, saying: "Of course, I look forward to amendments that will be made on full financial autonomy, which SNP members will be voting for.

"I suspect Mr Murray will be voting with the Tories as he worked so closely with them through two years of the referendum campaign."