The owners of the petrochemicals plant in Grangemouth, the future of which was in doubt last year, have won a $311m loan guarantee from the UK government to build a new facility to import shale gas.

Ineos said the project would protect thousands of jobs across the country and ensure the long-term future of the site. The loan guarantee will allow it to raise funds to invest in a new terminal at Grangemouth to import, store and process ethane from shale gas, as North Sea supplies dwindle.

The site was threatened with closure last year, but was saved after adramatic climbdown by the Unite trade union, which accepted drastic changes to staff working conditions. In return, Ineos said it had invested $405.8m as part of a long-term survival plan for Grangemouth to manufacture petrochemicals beyond 2017.

The company's 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 ethane tank will be the largest in Europe. It will be built by the specialist engineering company TGE Gas Engineering. 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.

The chief secretary to the Treasury, Danny Alexander, said: "Over $1.35bn of infrastructure projects have now been brought forward as a result of the UK guarantees scheme and $48.7bn worth of projects are pre-qualified.

"The Grangemouth guarantee is fantastic news for Scotland's economic future, and for the UK's energy security."