Labour's newest TD Patrick Nulty will vote against the Budget
The Dublin West deputy denounced the coalition's measures as "unjust" and claimed they would damage Ireland's chances of recovery.
Mr Nulty said the Budget would hit people on low and average incomes disproportionately, lead to higher levels of unemployment while attacking the marginalised, the sick, the elderly and young people with disabilities.
"The household charge combined with the increase in VAT is deeply regressive, unfair and ultimately counterproductive," he said.
"People on low and average incomes will be disproportionately affected while the wealthy benefit from flat rate taxation levied in this manner."
Just last week, Mr Nulty, 29, pledged support to Labour veteran Tommy Broughan after he left the Government benches over the renewal of the bank guarantee scheme.
Mr Nulty, elected to the Dail more than a month ago in the Dublin West by-election, had called for the Fine Gael/Labour Government to revise its plans for the austerity Budget to protect low and middle earners.
Announcing his intention to rebel against the Labour leadership, and face expulsion from the parliamentary party, Mr Nulty insisted there was an alternative to austerity - targeting the wealthy.
"Irish concentration of wealth is one of the highest in the EU-15," he said.
"Some 28pc of all wealth - housing and financial wealth - is owned by the top 1pc of adults. The Government should target this wealth."
Warning of thousands of job losses through capital spending reductions and public sector cutbacks, Mr Nulty also attacked child benefit cuts as "a very clear breach of the pledge" by Labour in the last general election.
Attacking other measures, including fuel allowance, health and disability cuts, he added: "I am not prepared to support measures which damage our economic recovery while attacking the weak, the sick, the marginalised and the vulnerable."
Last week, Tanaiste and Labour leader Eamon Gilmore said he was not surprised by Mr Broughan's opposition, but claimed he was confident no other backbenchers would revolt.
Mr Nulty is now certain to be the fourth Government TD to leave the coalition.
Alongside Mr Broughan, former Junior Minister Willie Penrose left over the closure of Columb Barracks in Mullingar while Denis Naughten was expelled from Fine Gael for refusing to support cutbacks at Roscommon hospital.
Mr Nulty accepted he would be kicked out of the party but insisted he would not resign.
He said the Government should have targeted the wealthy more instead of the vulnerable and poor.
"I think a really difficult choice would have been to ask those who are wealthy in our society to contribute more," Mr Nulty said.
"It's profoundly unfair. it cuts payments for young people with disabilities. it takes away the fuel allowance from our elderly people and it undermines people trying to get back to work."
Mr Nulty added: "I will not vote for those savings. I will not vote for reductions in the fuel allowance. I will not vote for reductions in back to school allowances for parents when there are clear and credible alternatives available."