Up to 650 jobs are expected to be cut at Falkirk -based Alexander Dennis as the firm reels from the fallout of the coronavirus crisis.
The jobs are at risk across three of the company's UK sites - at Falkirk, Guildford and Scarborough.
Scottish Labour leader Richard Leonard called for action as the country's economy "teeters on the precipice."
Union chiefs described expected job losses as a "devastating blow" for workers and their communities.
They said officials at Unite are working with bosses to minimise redundancies - part of a restructuring by the bus makers’ Canada-based parent firm NFI Group Inc.
Britain's biggest bus builder was bought by the Canadian giant last year for £320m.
The workforce were told today of a 45-day consultation with more than quarter of them are at risk of redundancy.
Union chiefs called on the UK government to bring forward £3 billion plans to get 4,000 new green buses on the roads.
They said the project would prevent job losses among the firm’s 2,500 strong workforce.
Unite national officer Steve Bush said: "Unite will be doing everything possible to support our members at this difficult time and we will be working with Alexander Dennis (ADL) to ensure the company explores all possible avenues to minimise redundancies.
“The coronavirus pandemic has impacted the automotive sector greatly.
"However, ADL is in a good position to take advantage of the need to reduce the carbon footprint of the UK’s passenger transport network."
The union warned Alexander Dennis bosses to avoid cutting jobs "opportunistically for the sake of short-term savings.
Bush added: “As Unite has emphasised in correspondence with minister for transport Grant Shapps, the easiest way to achieve this would be for the government to purchase the buses directly from manufacturers.
"The government could then lease them to the bus operators, many of whom are delaying orders because their cash flows have been impacted by the lockdown.”
Scottish Labour leader Richard Leonard is calling for the Scottish and UK Governments to intervene and create a Jobs Guarantee Scheme.
He said: “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.
“Our economy is teetering on the precipice. This is an industry with a bright future, but short term difficulties.
"It is of the utmost importance that the Scottish and UK governments take decisive action to protect jobs.
“Scottish Labour will continue to campaign for a Jobs Guarantee Scheme to provide quality jobs for all, as well as defending existing jobs such as at Alexander Dennis.
“Only a co-ordinated response from the UK and Scottish Governments, in conjunction with trade unions, will deliver for workers at Alexander Dennis.”
ADL chief executive Colin Robertson blamed the "unprecedented crisis" of coronavirus for the need to cuts jobs.
He added: "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."
A Scottish Government spokesman said efforts are ongoing to support jobs in Scotland.
He added: "This is a hugely challenging time for the whole economy and the bus industry is no exception.
"It will take some time for the sector to recover from the unprecedented impact of the coronavirus (COVID-19) pandemic on bus operator revenues – and the knock-on effect on bus manufacturing."
The spokesman said support is available for bus operators through the continued payment of the Bus Service Operators Grant and concessionary travel reimbursement at pre-COVID levels.
He added: "This is in addition to a £46.7 million emergency funding package we have provided to ramp up services as demand increases as we recover from the crisis.
"Through Transport Scotland and Scottish Enterprise, we are funding ADL to explore opportunities in bus retrofit solutions which could potentially assist with physical distancing on buses.
“We are working very closely with ADL to explore all possibilities to support jobs in Scotland and minimise the impact of COVID-19. We stand ready to assist any staff affected should job losses take place.”
