The Scottish Government failed to clearly estimate the full cost of seizing control of 11 key welfare powers from Westminster, the Auditor General has warned.
Caroline Gardner’s latest Scotland Acts report found officials had not clearly set out the total bill for putting the powers into practice.
Auditors warned it will cost “much more than £200 million (contributed by the UK Government) to implement” and that additional costs will have to come from the overall Scottish budget.
Implementing the new social security powers will be “hugely complex”, they added, and it will be “challenging” to ensure sufficiently skilled staff are in place to deliver them before the first wave of devolved benefits in summer 2019.
The report came days after Ministers were accused of being “under-prepared” for the handover, admitting the first wave of 750 jobs promised to Dundee as headquarters of the new Social Security Scotland will be housed at a yet-to-be secured “interim” office.
Ms Gardner said: “Putting the Scottish Parliament’s new financial powers and social security responsibilities into action is a huge and highly complex piece of work.
“More detailed workforce analysis and a much more transparent picture around overall costs are needed to ensure the right people and infrastructure are in place in time.
“Good early progress has been made on the government’s social security plans but they are now at a critical point.”
The report also notes new powers bring far greater budgetary risks as a result of the greater emphasis put on the performance of the Scottish economy.
Scottish Conservative social security spokesman Adam Tomkins said: “If ministers are caught out by these costs, the excess cash will have to come from the budget, and that will mean schools, hospitals and other public services being short-changed.
“Time is running out, and the people of Scotland will not forgive the SNP government if it makes a mess of the processing of this.”
Scottish Labour’s social security spokesman Mark Griffin said finding skilled staff for the new agency remains a “serious concern”.
He added: “Our public services are full of examples of the SNP government not being able to workforce plan adequately – that must not be replicated for Scotland’s new social security powers.”
Social Security Minister Jeane Freeman said she was “confident we are on track against our plans”.
She added: “We recognise the significant amount of work still to be done as we continue the programme to transfer powers, bring forward legislation and build the infrastructure we need.
“We are confident that we have robust plans in place and a full understanding of our overarching costs and are on track to deliver benefits to Scotland’s people. ”