It is believed Unite is to ballot 1000 members at the Grangemouth site for industrial action by the end of the week, but Ineos management told workers earlier this month any strike action could place their future and the company's future in jeopardy.
They claim industrial action will hinder their plans to bring millions of pounds of investment in, allowing them to modernise the site.
In a message to Ineos employees, works general manager Gordon Grant stated: "The facts are that at least 600 existing jobs are at risk if we cannot modernise. So the issue at stake here is the future of the site."
However, Unite representatives accused Ineos of using "bully-boy tactics" to try and prevent an inevitable ballot for industrial action over the company's current pension scheme proposals.
Pat Rafferty, Unite's regional industrial organiser, said: "We are in consultation with management, but at the same time they are making changes to the pension scheme.
"We have been backed into a corner and have no other option but to ballot members for industrial action to protect our pension scheme.
"They want to create a two-tier workforce with a pension scheme that is far inferior to the one currently in operation. There is no reason to make changes.
"We are moving to ballot our members for industrial action and make no apologies for it.
''It's in the company's best interest right now to resolve this dispute and they can sit down an resolve it tomorrow if they just back off the pension scheme."
Ineos have invested £100 million in the site since taking it over in 2005 and plan to invest a further £750 million in plant modernisation projects, stating the competitiveness of the site is critical to securing its long-term future – and pensions are only one part of the modernisation process.
An Ineos spokesman said: "Over a quarter of the entire money Ineos now spends on employees at Grangemouth goes into the pension scheme and independent pension experts say this figure could get close to 50 per cent.
"The amended pension scheme proposed by Ineos is very attractive. It will remain a final salary pension plan for existing employees and will still pay one 60th salary for very year worked, with the majority of costs still paid for by the company.
"The main difference is the current workforce will have to make a contribution to the scheme.
''This brings the Grangemouth workforce into line with nearly all UK employees.
"Ineos has proposed a nine month consultation period and we are only part way through that process. We are committed to a full and meaningful consultation period and recognise the topic of pensions is important to all members of staff."
Mr Grant added: "We want to do this right, that's why we are taking this time.
''What we are proposing is a good package for our people. We would be very disappointed if there was industrial action, given that we are some way through our consultation process. Industrial action would be painful to the company and individuals and is not in anyone's interest.
"We have no desire for it because it would significantly impair our ability to secure investment. If we don't have investment for these plants then in time they will stop and then jobs will go – that's the reality."