Opposition parties are demanding to know why £200m of money for public services in Wales is still being held by the UK treasury.
The £200m reserves have been carried over from previous budgets
The Welsh Assembly Government insists that it plans to spend the funds in the future.
The cash reserves are from money carried over from previous assembly government budgets.
But some opposition AMs say they are shocked to learn just how much money remains in treasury coffers.
The Liberal Democrat member of the assembly's finance committee, Jenny Randerson remarked: "I knew from my time as minister that some of the unspent money from previous years would be available to the assembly government for spending, but I had no idea it would be this much.
"It's time they opened the safe and spent some of this money. Rather than earning interest for the treasury, that money could be improving learning opportunities for Wales' children."
The revelation that the pots of money, known as 'End Year Flexibility' (EYF), had grown to £200m was made as the assembly's finance committee quizzed officials about the reserves.
The committee was told that the treasury holds £140m for spending on capital funding in Wales, which would include expenditure on projects such as renewing school buildings or hospitals.
In addition, a further £65m is available for in revenue funding, for paying the day-to-day costs of schemes and services in Wales.
Officials also stated that a further £300m was being held in London in a pot known as "non-cash reserves", which cannot be spent on public services.
A spokesman for the assembly government said: "The assembly government - in common with the other devolved administrations and Whitehall Departments - is able to carry forward resources from one financial year to the next so that wasteful end of year spending is avoided.
"This system, known as end of year flexibility, is a common sense approach to financial management and ensures that all available resources are put to use.
"It ensures that resources stay in Wales and don't return to the Treasury.
The Conservative's enterprise spokesman and chair of the finance committee Alun Cairns AM described the situation as "hypocrisy of the highest order".
"The fact that money has been sitting in the Treasury, when front line services have been experiencing cuts is alarming. How on earth has this money been accumulated?
"They accused John Redwood of sending money back to the Treasury when the assembly government has done five times as much."