The Grangemouth refinery could “relatively easily” be converted into a production facility for sustainable aviation fuel (SAF) over a few years, the trade union Unite has claimed.

General secretary Sharon Graham said she is “dismayed” by comments from the Scottish Secretary which appeared to “give up” on keeping the refinery open.

There are fears for about 400 jobs at the refinery after owners Petroineos said they would cease operations there in the second quarter of 2025.

Last week, officials from the company told a Holyrood committee it is simply too late to secure a smooth transition of the site from fossil fuels to renewables, as workers and politicians have suggested.

The Scottish and UK Governments have launched Project Willow, which aims to chart a future for the site in low-carbon fuels.

Unite says Grangemouth could be converted “relatively easily” to produce SAF – made from feedstocks such as cooking oil – saying this could happen in between one to three years without job losses.

They pointed to examples of other refineries around the world such as P66’s in Rodeo, California, which switched from crude oil to renewable feedstocks at a cost of one billion US dollars.

Unite are sceptical of Peroineos’ claims about the lack of financial viability at Grangemouth, saying there will soon be burgeoning demand for SAF.

In a letter to Energy Secretary Ed Miliband, Ms Graham said: “I do not need to remind you that this is the only refinery in Scotland and it is critical to our national security.

“A Labour Government not putting together a serious plan for the future of the site and the thousands of jobs which depend on it, would be a huge dereliction of duty.

“I was extremely concerned to hear the evidence of Iain Hardie and Colin Pritchard from Petroineos to the select committee on Wednesday. In particular, they claimed that no Government discussions around an intervention had happened yet.

“I was also dismayed at the comments of (Scottish Secretary) Ian Murray which seemed to give up on Grangemouth, which is totally unacceptable.”

Unite’s survival plan for the site involves the UK and Scottish governments taking a stake in the refinery to prevent its closure, though so far ministers in neither London or Edinburgh have suggested they would do this.

Last week, Mr Murray told STV “there is no way round” the fact the refinery is closing in May.

Iain Hardie, head of legal and external affairs for Petroineos, has told MSPs the site lost on average £385,000 per day last week and expects a £150 million loss this year.