Ineos set a deadline of 6pm for its employees to sign up to changes to pay, pensions and terms and conditions.
The company said yesterday that hundreds of workers had accepted the proposals, but Unite maintained that as the deadline passed, two out of three of its members had said no.
The company sent out a letter on Thursday to all 1,350 workers at the site asking them to indicate their rejection or acceptance of the plan.
Ineos said those who supported the survival plan would receive a transitional payment of up to £15,000.
The two sides have been embroiled in a bitter dispute for weeks, initially over the treatment of Unite convenor Stephen Deans, who was involved in the row over a selection of a Labour candidate in Falkirk, where he is chairman of the constituency party.
He was suspended, then reinstated, and is facing an internal investigation, which is due to report on Friday.
The dispute has since widened to the future of the entire site, with Ineos warning it will close without fresh investment and changes to pensions and other terms and conditions.
Jim Ratcliffe, the founder and chairman of Ineos, one of the world's largest chemicals manufacturers, said at the weekend that the fate of the Grangemouth plant rested with its staff.
"This is not a bluff. The clock is ticking," he said. "Grangemouth could have a future but that is absolutely in the hands of the workers. If we go down the wrong road, then I'm afraid this story will not have a happy ending."
Ineos said the plant, which has been shut down since last week because of the dispute, is losing £10 million a month.
It is believed that a meeting of the company's shareholders will be held tomorrow to discuss the dispute.
Unite said 65% of workers had so far rejected the company's plan, a figure the union believes will grow as more letters are returned.
Urging the company to return to talks at the conciliation service Acas, Pat Rafferty, Unite's Scottish secretary said: "This resounding rejection of the company's cynical blackmail sends a clear message to the company.
"The people who have so far rejected Ineos' ultimatum are the backbone of the plant, the people who keep the site running and the oil flowing.
"The people of Grangemouth and Scotland will be expecting Jim Ratcliffe and the Ineos shareholders to now take heed. Do the right thing tomorrow, drop the threats to the workforce, fire up the plant and get around the table at Acas.
"This is an overwhelming rejection of the company's blackmail and threats. This workforce has said that they want to secure a future for Grangemouth, free from fear, based on negotiation not confrontation.
"The shareholders at tomorrow's meeting need to recognise the strength of feeling among the workforce and instruct senior managers to get back to Acas and to come clean about the company's finances.
"We expect that number to grow and urge the bosses and the shareholders to see sense and get back around the table to secure a future for Grangemouth."