The Fiscal Advisory Council has said the Government's spending plans for the next budget are at the outer limit of what is prudent.

In a pre-budget report, the FAC says any more supplementary spending would be unwise.

Speaking on RTÉ's Morning Ireland, Fiscal Council chairman Prof John McHale urged the Government to be cautious. 

He said the 50% surge in corporation tax receipts last year was largely due to bringing onshore assets by multinationals, warning any changes in this sector could also lead to a slow down.

John McHale

"It does seem to be associated with the multinational sector," said Prof McHale. "Knowing the surge in corporation taxes really came about as a result of this onshoring of assets really just makes us more concerned that for the same reasons that it went up, changes in the multinational sector could lead it to go down.

"So we do need to be very careful."

On Government spending, Prof McHale said while there may appear to be some room for increases he warned: "One of the things I think we all want to make sure we don't do is repeat the kind of mistakes that got us into trouble before - and those mistakes were really made in good times.

"There is room for spending increases and once we reach our budgetary target a spending net of any adjustments to taxes can grow along with the underlying trend growth of the economy.

"So we're not saying that there shouldn't be any spending to deal with these very real pressures, but it's just about funding the appropriate balance."

In a pre-budget report, the FAC also says July's revision to the national accounts, which showed a 26% growth in GDP last year, has made the job of assessing budget plans extremely difficult.

The revised national accounts data, derided as "Leprechaun economics", have distorted all the key measures, such as GDP, GNP and the size of the Government's debt and deficit.

The FAC has had to make estimates using data on the domestic economy, which gives it a growth estimate for last year of 5% and not the official 26%. 

This in turn suggests the debt ratio is not 78% of GDP but a much higher 97% - emphasising the still high debt level the State carries, and the ongoing need to reduce debt.

The report says the 2015 data is "seriously distorted" by the activities of multinationals.

"The output and exports produced by these entities are adding to Irish GDP and GNP even though the production of the output takes place outside the State and no domestic labour is used in the production process.

"The current account of the balance of payments - an important indicator of imbalances in the economy - is also inflated.

"The latest data show an artificially high current account surplus of over 10% of GDP for 2015," the council said.

"The scale of this surplus is not consistent with the trade performance of firms producing output in the Irish economy or with the savings and investment patterns of domestic residents".

The council says the Government's budget plans comply with the fiscal rules - given the state of the economy as they estimate it to be - but warn against any more spending rises.

It says the planned budget 2017 spending rise of €1 billion of fiscal space, plus €1 billion in pre-committed spending carried forward from previous announcements, plus €500m in supplementary health spending announced in June means the actual spending increase will be in the order of €2.4 billion - much more expansionary than implied by the Summer Economic Statement.

The report states: "Weighing up these considerations, the Council assesses that the Government's current plans for a €1 billion package in Budget 2017, on top of a combined €1.4 billion of expenditure increases already committed for 2016 and 2017, is at the limit of the range of prudent policies.

"Any further relaxation of the fiscal stance in 2016 or 2017 beyond the current plans would not be appropriate given the strong pace of underlying economic activity, falling unemployment and the need to bolster the resilience of the public finances to adverse shocks.

"This means that current plans for 2016 and 2017 should be adhered to even in the event of tax revenue outperforming current forecasts by the end of 2016."

And it again warns against permanent spending increases funded by volatile corporation tax gains. 

It says the central growth forecasts for the next several years - and the estimates of fiscal space derived from them - face a number of potential risks. 

One of those risks - Brexit - has already occured. 

The FAC says that on current UK data the economic impact of Brexit should not materially impact on Budget 2017 but if there is a deterioration, such as the adverse scenario published by the National Institute of Economic and Social Research in the UK, based on the impact of uncertainty arising from the Brexit vote, then Irish potential economic growth could be reduced by about half a percentage point. 

This would in turn reduce the fiscal space estimated by the Department of Finance over the next five years by about one quarter to €8.5 billion.

Fiscal space is the estimate of money available for Government spending increases without new tax rises that is allowable under the fiscal rules.