SCOTLAND'S main parties have clashed over Holyrood's new powers ahead of a key vote on the Scotland Bill today.

David Mundell, the Secretary of State for Scotland, said Holyrood was "on the cusp of becoming one of the most powerful devolved parliaments in the world" and called on the SNP to spell out how much it would spend on benefits when substantial powers over social security are handed to MSPs.

The SNP, however, insisted the legislation was "far from perfect" and would give UK ministers "effective vetoes in many areas" over Scottish Government policy.

Scottish Labour claimed its pressure on the Government had strengthened the Bill, ensuring the "Vow" to devolve extensive new tax and welfare powers made by the three main pro-UK parties before last year's independence referendum had been fulfilled.

The bill, which is expected to receive Commons approval today, will transfer almost full control over income tax and responsibility for a £2.5billion welfare budget, including most disability benefits, to the Scottish Parliament in time for the 2017/18 financial year.

In addition, Scottish ministers will have the power to create new benefits in devolved areas and top-up UK-wide payments such as tax credits or child benefit.

Symbolically, the Scottish Parliament will be given new legal status as a permanent part of the the UK constitution.

Mr Mundell said: "It is now time to find out what the price tag on benefits could be for Scottish tax payers.

"The vote in the Commons will mean the Scottish Government have run out of excuses for not telling Scotland what their benefits plans are.

"People now want to know how much this is all going to cost and how it will be paid for."

His comments were echoed by Ian Murray, Labour's shadow Scottish secretary, who said:

"The challenge now is to the SNP Government to tell us how they are going to use these powers."

A spokesman for First Minister Nicola Sturgeon hit back: "The Scotland Bill is far from perfect, giving Scotland responsibility for just 14 per cent of welfare spending.

"Last week, after months of pressure, the UK Government lodged over 80 amendments to try and bring the Scotland Bill up to scratch.

"These are welcome, but there are still shortcomings, and Holyrood will make a final decision on the Bill in a few months’ time."

He repeated the First Minister's warning that the SNP would only rubber-stamp the bill if the so-called fiscal framework, which will adjust the Scottish Government's budget to take account of the changes, is "fair".?

The SNP has tabled a last minute amendment calling for full fiscal autonomy - the devolution of complete control over taxation and spending in Scotland.

The Nationalists have also demanded the power to call and a second independence referendum.

First Minister John Swinney said: "On this issue of this significance the Scottish Parliament should be able to determine exactly how this issue is handled in the years to come."

But Alex Johnstone, the Scots Tory MSP, said: "Handing the SNP the opportunity to unilaterally call separation referendums would be chaotic and cause a great deal of uncertainty."