Pilots and cabin crew at Alitalia have rejected the deal offered by CAI to save the bankrupt carrier.
The unions said they would take any action to cancel an agreement signed on October 31 with CAI, a 16-strong consortium of businesses bidding for the airline.
All nine unions representing workers at the national airline agreed in outline to the CAI rescue plan in September.
But since then only four have signed new deals on work contracts.
The other five, representing more than 1,000 the pilots and flight staff, met this week and decided to oppose the work contracts.
They said they would take "any action needed" to secure their demands which some have taken to mean strikes. The unions have so far declined to say exactly what actions they might take.
CAI has also submitted its offer for Alitalia to the airline's administrator, Augusto Fantozzi who has told it that it will remain "binding and irrevocable" until November 30.
Under its bid, CAI will buy the government's 49.9% stake in Alitalia and keep the profitable bits of the airline. The unprofitable bits will be liquidated.
It will then merge with rival Italian carrier Air One to form a new airline.
The scheme will lead to 3,250 redundancies which have proved.
CAI has also been in talks with Air France KLM and Lufthansa about taking a minority stake in the new carrier and with BA over a possible commercial agreement.