The Scottish Government is to add a further £66m to the country's cash reserves to help against "future budget volatility".

Finance secretary Derek Mackay said the investment, which is part of a £453m underspend from the 2017-18 budget, was a "prudent" choice.

He added that funds raised from devolved taxes had exceeded forecasts by £50m in the last fiscal year.

Scottish Labour slammed the finance secretary for "gross mismanagement", saying such a high underspend while ministers inflicted cuts on councils was "reckless".

The Scottish Tories said the underspend was the result of a projected tax revenue shortfall caused by Scotland's "economic underperformance".

Mackay gave of the provisional cash outturn for 2017-18 in a statement at Holyrood.

He previously detailed plans to carry forward £235m to 2018-19, while a further £100m comes from Treasury cash given to the Scottish Government to set up its new social security agency.

This is in addition to the £50m of higher-than-forecast tax receipts from the Land and Buildings Transactions Tax and Scottish Landfill Tax.

Mackay said: "The 2017-18 provisional outturn results show that once again the Scottish Government has prudently and competently managed Scotland's finances ensuring our public services are provided for in a fiscally responsible manner.

"The successful implementation of new financial powers has been delivered against the backdrop of uncertainty around the UK's exit from the EU and the UK Government's continued austerity measures.

"Under the current devolution settlement, the Scottish Parliament is not permitted to overspend its budget.

"As a consequence, we have consistently adopted a position of controlling public expenditure to ensure that we live within the budget control limits that apply.

"These underspends represent a fraction of our overall budget, but will be carried forward to support this year's budget and increase our reserves to ensure we can respond to future challenges."

But Labour said the cash was a "slush fund" to bribe the Greens to help the Scottish Government pass its budgets.

The party's finance spokesman James Kelly said: "SNP finance secretary Derek Mackay underspent almost half a billion pounds whilst cutting council budgets by £1.5bn.

"Money is piling up in his slush fund for his annual bribe to win Green votes to get his budgets passed.

"This is simply a gross mismanagement of Scottish finances by a Finance Secretary that is clearly out of his depth.

"Throughout his tenure as finance secretary, Mr Mackay his tinkered on tax instead of ensuring the richest pay their fair share to fund our schools, hospitals and other vital public services.

"Derek Mackay must apologise to communities across Scotland that have been damaged by his reckless actions."

Scottish Conservative shadow finance secretary Murdo Fraser said: "Derek Mackay might like to fool us all into thinking this £453m underspend figure is an insignificant sum.

"But it's worth noting that it's higher than what the SNP's independence blueprint said it would cost to create a separate state.

"And the finance secretary is having to put money aside to meet a projected shortfall in tax revenues due to Scottish economic underperformance.

"If the SNP could grow the Scottish economy at least at the same rate as the rest of the UK, there would be hundreds of millions more to spend on public services and reduce the tax burden on hardworking people and businesses."