In another dramatic day in the Grangemouth dispute that has threatened both fuel supplies in Scotland and the long-term future of the chemicals and refinery complex, Unite said Ineos's announcement amounted to a "sign or be sacked" ultimatum.
It is considering legal action over what it described as "a gun to the head" of the plant's workers.
First Minister Alex Salmond left the SNP conference in Perth to hold talks in Edinburgh with both sides in an attempt to forge a deal.
After the meeting, Mr Salmond called for Unite to swiftly confirm their willingness to provide a guaranteed period of normal working to New Year's Eve at Grangemouth.
He also called on Ineos to fulfil their parallel commitment to fire up the plant. Mr Salmond said: "Whilst there will be a long way to go, I believe that acceptance of my proposal tonight would be an essential first step in this process."
There was little sign of panic-buying at petrol forecourts despite Ineos confirming the plant which refines North Sea oil supplies for distribution in Scotland will remain shutdown until Tuesday at the earliest.
Fuel retailers insisted motorists were heeding advice that contingency plans were in place to ensure fuel shortages could be averted.
Analysts said supplies were unlikely to run out even if the dispute carried on beyond Tuesday.
Nick Vanderwell, of the UK Petroleum Industry Association, said: "Ineos has been stockpiling fuel and there is excess gasoline in Europe that could be imported if necessary to cover any shortfall."
Ineos upped the stakes on Thursday by pulling out of arbitration talks with Unite, even though the union cancelled a planned 48-hour strike, and saying it would submit "survival plan" proposals for the plant, which employs around 2500 staff and contractors, directly to its workforce.
Details of the proposals were made clear in a letter to staff yesterday and include ending the final salary pension scheme, freezing pay and bonuses, reducing shift allowances and involving new agreements with unions, including having part-time conveners.
Unite said workers were being told to accept the new terms and conditions by 6pm on Monday in advance of a company shareholders meeting on Tuesday to decide the plant's future.
The union's Scottish secretary, Pat Rafferty, said: "This is cynical blackmail from a company that is putting a gun to the heads of its loyal workforce to slash pay, pensions and jobs. We are considering taking legal action over the company's menacing tactics and urge members not to be threatened into signing their livelihoods away.
"Menaces and threats will not secure the future of Grangemouth which rests in the hands of a few unaccountable shareholders."
Ineos said it had decided to appeal directly to the employees.
Chairman Calum MacLean said: "This is D-Day for Grangemouth. If we can get the changes we want, the company has committed to investing a further £300 million in the site which will help secure its long-term survival.
"The shareholders have expressed extreme concern that the action over recent days has cost the site £20m at a time when losses are already £10m per month. The site cannot afford this, hence the urgent need for employees to decide to support the company."