SIPTU president Jack O'Connor warned that Labour's support of the controversial Aer Lingus sale will be considered carefully by his members before deciding on whether to back the party ahead of the next general election.
Ms O'Connor insisted the current offer from airline giant IAG raised "serious concerns" for his members, who fear the company's boss Willie Walsh will seek to outsource jobs once the deal is sealed.
The warning follows angry clashes in the Dáil as Opposition politicians accused the Government of seeking to sign the deal into law without proper scrutiny or debate.
Meanwhile, the Impact trade union criticised the "unseemly party atmosphere" around the sale and called on Aer Lingus management to give more assurances to staff.
"Workers woke up to a circus atmosphere with Willie Walsh in town to sell his bright shining new deal," Impact spokesman Niall Shanahan said.
"The Government has given its seal of approval and we are 24 hours away from a vote that will seal Aer Lingus's fate," Mr Shanahan added.
Labour believes the unions are posturing and will eventually agree to the improved deal offered by IAG, according to a senior party source.
However, Mr O'Connor said his union's backing for Joan Burton's Labour Party at the next general election will be decided at Siptu's conference in October and the Aer Lingus sale will influence that decision.
The union's support could prove crucial for Ms Burton's future leadership as the party struggles to regain public confidence after four years in Government with Fine Gael.
After months of wrangling over the deal, Labour agreed to sign off on the €335m sale of the State's remaining share in what was once a prized asset. Labour TDs, who vehemently opposed the initial offer tabled by IAG, backed down and agreed to support the new deal, which offered guarantees on jobs and on the vital landing slots in Heathrow Airport.
However, Clare TD Michael McNamara faces being ejected from the Labour Parliamentary Party if he votes against the deal today. Mr McNamara told party colleagues yesterday that he is still undecided as to whether he can support the package proposed by IAG.
His name was included on a statement issued on behalf of the so-called 'Aer Lingus Seven' group of Labour TDs on Tuesday night supporting the deal.
However it emerged yesterday that his name was included without his permission.
A senior source tonight said Mr McNamara has sought further assurances surrounding the guarantees over the slots but that he will be booted out of the party if he opposes the deal.
Announcing the new package, Transport Minister Paschal Donohoe revealed around 50 jobs, mostly in administration, will be lost at company's headquarters in north Dublin next year.
However, he pledged the takeover would result in more than 600 jobs in the next five years.