The owners of the giant terminal at Grangemouth have secured a £230 million loan guarantee from the UK government to build a new facility to import shale gas.
Ineos said it was one of the most important infrastructure projects of recent times in Scotland and would protect thousands of jobs across the country.
The company said confirmation of the loan guarantee would allow it to raise funds to invest in a new terminal to import, store and process ethane from shale gas in Grangemouth as North Sea supplies dwindle.
The future of the site was in doubt last year because of an industrial dispute, but Ineos said it had invested £300 million as part of a long-term survival plan for Grangemouth to manufacture petrochemicals beyond 2017.
Chairman Jim Ratcliffe said: "Without doubt, this is one of the most important projects of recent times in Scotland, with implications to be felt right across the UK, not only for employment but also for manufacturing in general.
"Our ability to import US shale gas underpins the future of manufacturing at Grangemouth and across many businesses in Scotland. It is a vital step towards preserving the long term future of the Grangemouth site and those businesses that depend upon its continued presence in Scotland."
The loan guarantee from the UK government enables Ineos to raise financing on £230 million specifically to cover the import facility and storage tank to be built at the site.
Chief Secretary to the Treasury Danny Alexander said: "Over £1 billion of infrastructure projects have now been brought forward as a result of the UK guarantees scheme and £36 billion worth of projects are pre qualified.
"Our action is creating the right conditions for more investment in our infrastructure, helping to build a stronger economy and a fairer society across the country. The Grangemouth guarantee is fantastic news for Scotland's economic future, and for the UK's energy security."
Ineos said the announcement was a major step forward that ensured the long-term future of petrochemical manufacture at Grangemouth.
The ethane tank will be the largest in Europe and is central to the company's plans to import shale gas from the US. By 2016 Grangemouth will be a shale gas-based facility, which Ineos said was essential if it is to compete in world markets beyond 2017.
Chief finance officer Gerry Hepburn said: "The news that the UK government is supporting our plans to build an ethane import and storage facility at Grangemouth with a loan guarantee is a critical element to ensure the long term future of the site. We will use the loan guarantee to raise funds through a public bond issue.
"The proceeds of the bond will be used to fund the ethane tank project at Grangemouth. It will be very rewarding to see the renewal of the site starting to take shape as we begin construction work."
Ineos said it had finalised contract agreements with specialist engineering company TGE Gas Engineering for the construction of the storage tank.
TGE managing director Werner Schlott said, "We are very pleased to have been awarded the contract to build the ethane storage tank for Ineos. As a company we have extensive expertise and knowledge of tank construction and relish the opportunity to bring this to Scotland."
Scottish Finance Secretary John Swinney said: "The Scottish Government welcomes the announcement that loan guarantee funding has been secured and that significant new investment planned at the Grangemouth petrochemical plant will proceed.
"We are supporting these investment plans with £9 million Regional Selective Assistance funding to secure the future of the Grangemouth facility and safeguard jobs in Scotland.
"We remain committed to working with all partners involved to ensure a thriving, sustainable plant at Grangemouth, and the new import terminal and ethane storage facilities is a key step in securing the long-term future of the site."
A Unite spokesman said: "The confirmation of loan guarantees is obviously a welcome move and is part of the package which also saw the workforce make great sacrifices to secure the future of Grangemouth.
"Grangemouth's future doesn't just rest on securing investment, but also on the goodwill and hard work of a skilled workforce.
"Over the coming months it is vital that Ineos engages with its workforce and Unite in a positive manner to ensure there is the skilled manpower to deliver Grangemouth's future."