Fine Gael has accused Fianna Fáil of "pursuing populist and uncosted economic policies that could bankrupt the country again", according to an internal Fine Gael memo seen by RTÉ News.
Echoing a political charge made in the Dáil by Taoiseach Leo Varadkar, the document contains the accusations that the main Opposition party is engaging in "reckless spending calls that are growing louder with every passing week".
It claims that spending pledges made by Fianna Fáil in the first five months of 2018 amount to €1.5bn.
"Michael McGrath speaks of fiscal prudence and yet he allows his Fianna Fáil colleagues run wild with hundreds of millions in extra spending being promised every week to every interest group. None of these promises can be trusted, this is dishonest politics," it states.
However, in a clear indication of acute tension between the two parties, Fianna Fáil has labeled the charges made by Fine Gael as "nonsense" and "another attempt to get their divisive Tory-style agenda back on the table in advance of the coming budget negotiations".
Under the confidence and supply deal, Fianna Fáil has committed to supporting the Government from Opposition for three Budgets, the third of which is due in October.
Back on 25 April, the Taoiseach challenged Micheál Martin during Leaders’ Questions about various spending pledges the Fianna Fáil had made.
At the time, Mr Varadkar said: "What is wholly dishonest in politics is going around the country promising every interest group that you will do everything they ask and you will do it this year, because that is not possible."
TROUBLE IN PARADISE?: As @MichealMartinTD & @campaignforleo have a spat over spending priorities, @MaryLouMcDonald claims "There's trouble in paradise. The Government parties have fallen out with each other here before our very eyes." pic.twitter.com/wWNO10xZhO— RTÉ Politics (@rtepolitics) April 25, 2018
RTÉ understands that after this spat the Taoiseach set up an internal Fine Gael working group to start costing Fianna Fáil proposals.
The document accuses Fianna Fáil of "roulette wheel economics" that will "repeat the mistakes of the past".
It claims that Fianna Fáil has made spending calls amounting to over €1.5bn for first five months of 2018.
Quoted in the internal Fine Gael document, Longford-Westmeath TD Peter Burke has challenged Mr Martin "to produce figures to back up party’s political rhetoric". The claims about Fianna Fáil’s spending pledges have not been independently assessed.
A spokesperson for Fianna Fáil dismissed them as "a juvenile attempt by Fine Gael to distract from their own failings in health and housing".
The document claims that the €1.5bn figure arrived at following "a thorough investigation is a minimum and conservative estimate, because it does not include many more spending proposals made by Fianna Fáil in recent months that are impossible to cost".
It claims there are 25 further spending calls that Fine Gael has been unable to cost.
"For example, we have not included Thomas Byrne’s call for significantly more money for capital expenditure in education," it states.
The Fianna Fáil spending proposal figures used by the Fine Gael working group are detailed in its party press releases and in private members’ bills and motions since January.
Mr Burke said: "We are only now seeing the benefits of recovery with incomes and employment growing, but this type of irresponsible political grandstanding from Fianna Fáil is exactly what brought the country to its knees ten years ago.
"They are experts in the field of boom to bust policies and trying to be all things to all people.
"I am directly challenging Micheál Martin and his party to stop shouting from the sidelines and to produce real figures to back up reckless spending calls that are growing louder with every passing week."
Fine Gael claims the various Fianna Fáil spending demands would cost taxpayers at least an extra €1,535m in total every year.
Examples of the spending calls include €200 million to support suckler farmers; €163 million additional spending on roads and €120 million to reverse FEMPI cuts to GPs.
Mr Burke also claimed: "This is roulette wheel economics of the very worst kind. Hoping your numbers will come up is not socially responsible Government."
Responding to details of the leaked document, Fianna Fáil said in a statement: "Not only is this a juvenile attempt by Fine Gael to distract from their own failings in health and housing, it’s shockingly ironic for Fine Gael to question the costings of any party’s policy agenda when that party is responsible for the single most expensive election promise in the history of the State when they promised to abolish the USC before the last election.
"Fine Gael has previously stated that they believe in a US-style tax regime, where the wealthiest in society are prioritised for tax cuts and those who rely on decent public services can wait.
"But Fianna Fáil and the Irish people gave them their answer in 2016 and we have forced them to have proper investment in services in subsequent budgets.
"Fianna Fáil makes no apologies for wanting better public services, including better funded schools and hospitals with appropriately paid teachers, nurses and doctors.
"We believe in State investment in public and affordable housing and remain loyal to providing a decent quality of life for our people.
"Fine Gael can continue to focus on silly politics and their spin over substance approach to Government. Fianna Fáil will continue to focus on using our mandate to get things done and improve the lives of our people."