As a furious war of words escalated, Ineos said in a statement: "Unite has refused to accept our offer to restart the Grangemouth plant in exchange for a commitment that there will be no further industrial action this year.
"The refusal occurred earlier this morning following a face to face meeting between the company and the union.
"Ineeos needs the undertaking because it would be hazardous to start up the plant only to have to stop it a few days later in response to further threats of industrial action.
"The company will now focus on completing the 60-day consultation period with its staff on the Ineos Survival Plant."
Calum Maclean, Grangemouth chairman said: "Unite's response is unbelievable, given how much effort has been put into securing this deal with ourselves, the Scottish and the UK governments all working hard to find a way forward. We will now concentrate on discussing the Survival Plan with our staff during the 60 day consultation."
The Ineos survival plan lays out the critical changes that are required to ensure that Grangemouth remains competitive and suitable for further investment.
Earlier, the bitter dispute took another twist after union leaders urged HM Revenue and Customs to investigate the tax affairs of the site's owners. Unite said it had concerns that Ineos Group's arrangements "obfuscate the true position" of its activities.
The two sides are locked in a row which started over the treatment of a union convenor but now involves the future of the site.
The company has written to workers asking them to agree to changes to pensions and other terms and conditions as part of its survival plan to prevent the site's closure.
Unite said the move amounted to an ultimatum for workers to agree to worse pay and conditions by 6pm on Monday or face the sack.
Scotland's First Minister Alex Salmond held separate talks with the two sides last night in an effort to break the deadlock, which has led to the site being shut down.
Ineos said it would restart the Grangemouth site if the company received a guarantee that there will be no further industrial action of any kind during the 60-day consultation period.
Writing to Chris Davidson, head of the anti-avoidance group for the HMRC, Unite general secretary Len McCluskey said: "The current uncertainty as to the true nature of the company's UK activities is causing genuine confusion, not least to the Ineos employees we represent, as well as other stakeholders.
"Analysis of the company's accounts is in itself made difficult by the sheer number of reorganisations undertaken by the Ineos Group. However, analysis of the latest 'Chemicals' accounts (2012) shows a fundamental paradox worthy of investigation at the heart of the company's accounting position.
"My union has employed expert external analysis for the purpose of trying to better understand Ineos at a time of great uncertainty. On the back of our findings I now urge the HMRC to launch a formal investigation into the affairs of Ineos."
The company said the dispute had cost £20 million, on top of monthly losses of £10 million. It rejected Unite's claims over its finances.
Unite's Scotland secretary Pat Rafferty said: "Unite has said all along that there is absolutely no need for the extreme shutdown that Ineos has imposed on Grangemouth.
"We offered three days ago at Acas to withhold industrial action until the end of the year if Ineos would restart the plant immediately. That offer still stands."