Bus builder Alexander Dennis Limited is expected to cut 650 jobs due to the coronavirus pandemic.
The Falkirk-based firm employs 2,300 people, including 1,000 at its Scottish factory and head office.
The workforce were told on Monday that more than quarter of them are at risk of redundancy, following a 45-day consultation.
Britain's biggest bus builder was bought by a Canadian company last year for £320m.
But since the lockdown passenger numbers and, as a result, orders for new buses have plummeted.
ADL chief executive Colin Robertson said: "A few short months ago, we were looking at a record year of sales, reaping the benefits of all our investment in new technology, new products and new markets.
"All of this changed with the impact of Covid-19."
Despite the support offered by the UK government's job retention scheme he added that industry is facing an "unprecedented crisis".
Managers in Falkirk have been appealing to the Scottish and UK governments to step in and save thousands of industry jobs, citing the prime minister's commitment to renewing Britain's bus fleet with lower-emission models.
But no special deal for the sector has been agreed.
Unite Scotland described the news as "devastating" for the workforce and their families.
Regional officer Willie Thomson said: "We believe these actions are premature and urge the company to continue to explore all options to protect jobs and livelihoods.
"We stand ready to work with them to achieve this.
"Covid-19 has had a significant impact on the bus industry with the collapse of new orders from operators due to the crisis."
'Cleaner and greener'
The union also called on both the Scottish and UK governments to support the industry.
Mr Thomson added: "The Alexander Dennis workforce has a crucial role to play in a cleaner and greener future for both our economy and our communities.
"We will be fully engaging with the company in the coming weeks and we will do all we can to save these jobs.
"Each job is a worker, a family affected, and they shouldn't be the ones to pay the price for this crisis."
Scottish Labour leader Richard Leonard said the industry was one with "a bright future, but short-term difficulties".
He added: "This news is deeply concerning and any redundancies following from this will be a hammer blow, both to the workforce and their families as well as Scotland's manufacturing base.
"That's why no stone should be left unturned to save these jobs."
Scottish Conservative Central Scotland MSP Alison Harris said: "It's essential both the UK and Scottish governments work together to try and save as many of these jobs as possible, and ensure the company can continue its pioneering work in the future."
ADL's head office is in Larbert and its main factory is in Camelon.
It also employs workers in Guildford and Scarborough.
Alexander Dennis, which is the world's largest producer of double-decker buses, was acquired by a group of Scottish investors in 2004.
Stagecoach founders Sir Brian Souter and Dame Ann Gloag owned more than half the company, which is now part of the NFI Group, which makes buses for the North American market.