Some workers at the Grangemouth refinery have been sent redundancy letters by their employer Petroineos.

The site, which is Scotland’s only oil refinery, is set to close by the summer with the loss of more than 400 jobs.

It is understood the majority of workers have agreed voluntary redundancy deals, and job losses will be spread out over the next 18 months.

Union leaders had hoped that the central Scotland facility could remain open for longer to provide time for a greener fuel alternative to be established at the site.

The Unite union said the move was a “national disgrace” and accused the UK and Scottish governments of being “missing in action” as the closure takes effect.

Petroineos said the closure of Grangemouth was due to it being unable to compete with sites in Asia, Africa and the Middle East.

The company said the decision would “safeguard fuel supply for Scotland” by converting the site into a terminal able to import petrol, diesel, aviation fuel and kerosene into Scotland.

The shift is expected to keep around 65 of the roughly 500 jobs on the site.

Sharon Graham, general secretary of trade union Unite, said the closure was a “needless act of industrial vandalism”.

“There is a clear plan for the future of Grangemouth in place,” she said. “Yet the UK and Scottish governments are missing in action.”

Ms Graham suggested Grangemouth could be used to produce sustainable aviation fuel.

“Just last week the UK government announced the expansion of Heathrow and said that sustainable air fuel (SAF) will be needed,” she said.

“Well, Grangemouth can produce SAF – where is the joined up thinking?

“This the moment for the government to lead and stand up to Petroineos in the national interest. Petroineos must allow the transition to SAF and biofuels.”

Workers held a rally at the refinery last summer in protest at the plans

Petroineos said it expected the new distribution hub for finished fuels to open by early summer.

A spokesman said: “The process of safely transitioning the site from a refinery into an import terminal is under way and will continue over a number of months.

“Fully winding down and dismantling the refinery is anticipated to take around five years in total, so the new terminal business will operate in parallel with this process.”

Deputy First Minister Kate Forbes said it was “deeply regrettable” that Petroineos had issued redundancy letters to the workforce.

She said: “The Scottish government believes that refining at Grangemouth should continue, and a premature closure of the refinery is fundamentally short-sighted and detrimental to the transition to net zero.

“We stand ready to support everyone impacted and have announced dedicated skills support for those facing redundancy. And we will continue to do everything we can, within our limited powers, to secure a long term future for Grangemouth.

“However, the UK government has so far failed to live up to its commitments or offer any assurances about the future of Grangemouth and its workforce.”

She urged the UK government to bring forward investment to save jobs at the refinery.

The UK government’s Department for Energy Security and Net Zero said it was “leaving no stone unturned to find a viable long-term future for Grangemouth”.

A spokesperson added: “Before July, there was no overall plan for the future of the Grangemouth refinery.

“Within weeks, we worked with the Scottish government to put together an unprecedented £100m package to support the community and invest in the local workforce, along with tailored support to secure good, alternative jobs.”

The spokesperson said it was providing £1.5m of joint funding for Project Willow which is examining how to create a low-carbon fuel and chemicals hub at Grangemouth.

The Grangemouth refinery was opened by BP in 1924 and expanded into petrochemicals in the 1950s,

It is the main supplier of aviation fuel for Scotland’s airports and a major supplier of petrol and diesel ground fuels across the Central Belt.

Ineos acquired the site in 2005 and is responsible for the entire plant, while the the refinery itself is owned by Petroineos – a joint venture between Ineos and PetroChina.