The owner of the Wrightbus factory has said he has not been able to reach a deal to sell it to a new owner.
However, the union representing workers at the County Antrim bus manufacturing firm said a deal was still be possible.
“It is not dead in the water – it’s hanging by a thread,” said Unite official George Brash.
Former Wrightbus director Jeff Wright said on Thursday the sticking point was over farmland that he does not consider to be part of the factory site.
He said failure to complete a deal was “deeply regrettable, especially after the exhaustive efforts all of us involved have gone to in providing every possible support”.
The Ballymena business was placed into administration and put up for sale last month.
The Wrightbus premises are owned separately from the manufacturing business and held in a company called Whirlwind Property Two, which is not part of the Wright group and is therefore not under the control of the administrator.
The property company is controlled by Mr Wright, the former owner of Wrightbus.
English industrialist Jo Bamford wants to buy the Wrightbus business and the factory through his Ryse Hydrogen company.
Mr Wright said he hoped the firm’s administrator Deloitte had “not been sidetracked by the Bamford engagement” and could work with the remaining bidders.
Mr Brash of the Unite union said he understood an outline deal had reached on Wednesday but changes were made my Mr Wright on Thursday morning, which “threw the entire negotiations into uncertainty”.
He added: “Unite has sought to engage directly with Jeff Wright but as yet our request for a meeting has gone unanswered.
“We are calling on all sides, in particular Jeff Wright, to do the right thing and adopt a sensible approach that will open the door to a future for these workers.”
‘Paisley role unhelpful’
On Thursday morning, Democratic Unionist Party (DUP) MP Ian Paisley said: “Today is decision day.”
“If the answer is that we cannot get the deal consummated, in terms of the land sale deal, then I’m afraid the worst of all situations prevails and we’re left with liquidation of assets of the company,” he said.
“It’s either the continuation of building the best buses produced in the world or it’s the end of bus building in Ballymena.”
The North Antrim MP said Mr Bamford told him on Tuesday that jobs would be created within two weeks if a deal was made.
“The Wright family have made a mammoth contribution to Northern Ireland over three generations and this maintains that rich history,” said Mr Paisley.
“Everyone needs to put their shoulder to the line and get this deal over the line.”
But Jeff Wright said Mr Paisley’s role had been “unhelpful to say the least” and he advised that the MP to “leave the business of deal making to the professionals at Deloitte”.
“In what could be considered a vote-campaigning exercise, Mr Paisley continually championed Mr Bamford throughout this process,” added Jeff Wright.