Ministers have no clear understanding as to how massive spending cuts slated for the next Parliament will be delivered, public finance experts have warned.

Renowned economics think-tank the Institute for Fiscal Studies claimed in its post-Budget briefing last week that departments outside the ringfence protecting the NHS, schools and overseas aid budgets, face average annual spending cuts of 6.1% from 2015/16 to 2018/19 – compared with only 4.6% cutbacks over the current spending round period.

IFS director Paul Johnson said the ‘overwhelming fact' about spending plans remained that spending in unprotected departments – including local government budgets – is set to have decreased by more than a third by 2018/19 – with most of the cuts yet to come.

‘The chancellor effectively expressed a lot of confidence in his ability to deliver these cuts,' Mr Johnson said. ‘He did not take the opportunity to ameliorate them,' he added.

According to the IFS director, the Budget ‘leaves us with as little sense as we had before of quite how the very large public spending cuts still in the pipeline will be actually delivered'.

Mr Johnson accused the chancellor of using conjuring tricks to disguise the impact the long-term and upward costs public service pensions would have on the public finances.

Ahead of last week's Budget, ministers had announced that in light of the Hutton reforms, the Exchequer had shifted £1bn in extra pension contributions from Annually Managed Expenditure – which includes payment on welfare, pensions and debt interest – to cash pots covering the annual running costs of public services, known as Departmental Expenditure Limits.

The IFS director said: ‘The Treasury is, probably, sensibly "charging" spending departments for these increased costs.'

‘Because it is taking money from them, it is saying it has extra money to spend,' Mr Johnson said. ‘Hey presto!'

He accused the Treasury of finding ways to spend extra money now at the risk of worsening public finances in the long run.

‘That is not a sensible way to think about fiscal policy,' Mr Johnson said.

Gemma Tetlow, IFSprogramme director, said the Government had fallen into bad habits by sanctioning permanent giveaways funded by temporary revenue raising measures, which she said could endanger long-term finances.

She also found fault with a number of recent spending commitments, buttressed with either no or only temporary extra money.

These included the £800m cost of free school meals announced in December's Autumn Statement, and commitments from the 2013 Budget, such as £1bn annual costs of implementing Dilnot report on care funding and £3.7bn costs of higher public sector National Insurance Contributions (NIC).

Local government faces a £600m NIC hit as a result of the move to end contracting out into direct benefit pensions, something council chiefs are increasingly worried about.

However, Ms Tetlow said the introduction of a welfare cap, set at £119bn in 2015/16, out of that year's total £219bn bill welfare expenditure, could limit some of the negative risk on public finances.