Scottish Secretary David Mundell has accused the Scottish Government of not "coming clean" on its welfare plans.

Ahead of the Scotland Bill debate in the Commons, Mr Mundell challenged Scottish ministers to set out how they intend to use new powers, in pounds and pence.

The SNP hit back warning that cuts being planned by his Conservative government showed Edinburgh needed greater control over the issue.

The Conservatives plan to cut an extra £12 billion from the welfare bill.

The pledge was made in the run up to last month's General Election but has been reiterated in recent days by David Cameron.

Last night Mr Mundell said that the planned reductions in welfare spending were designed to make sure that "we don't have to ask working families to pay more".

Scotland's only Tory MP said that Scottish ministers would instead have to "justify (any extra welfare) spending to the hard-working men and women in Scotland. who will be paying for it in their wages every month".

He predicted that it was "crunch time" for the SNP Government.

"They will soon be receiving the powers over welfare which they have long wanted; they now have to tell us how they intend to use them," he said.

"People deserve to know the price tag of their policies and who will be picking up the bill."

A Scottish Government spokesman said that £12 billion-worth of planned social security cuts showed the "need to create a fairer" system.

He warned that the extra cuts being proposed would "only impact further on some of the most vulnerable people in our society including children".

Both Labour and the SNP called on each other to back their proposed amendments to the Bill today.

The SNP proposals would give MSPs greater powers over working age benefits and back to work schemes, National Insurance, and employment law.

Dr Eilidh Whiteford, the SNP's social justice spokesman, said that the welfare elements of the Scotland Bill fell "far short even of the spirit of the Smith Commission, as well as its letter".

Labour's Shadow Scottish secretary Ian Murray will also accuse the Conservative Government of not abiding by the "letter and spirit" of the recommendations of Smith, set up in the wake of last year's independence referendum.

He called for the Scottish Parliament to have the final say on benefit rates in Scotland.

Last night the SNP hit out at plans to devolve a large proportion of income tax to Holyrood.

SNP MP Pete Wishart called the move a "half way house".

Tory MP Sir Edward Leigh called for all of income tax to be handed over to MSPs saying that the move "makes logical sense".

"I believe that what we are giving the Scottish Parliament at the moment is only half a power", he told MPs.

He also warned that Westminster could "cut the ground from beneath the feet" of Holyrood, by raising the thresholds at which people start to pay the levy, an area over which MSPs would have no control.

Another Tory MP Nigel Mills called on ministers to "step back" and look at the case for a federal system, so people "will understand what tax they are paying to what parliament and what they will get out of it".

He called the current proposals a "strange half-way fudge we may regret in a few years' time".