Unite the Union's Regional Secretary Jackie Pollock has confirmed Wrightbus has entered into administration.
Unite the Union said it is calling on the British Government to intervene and save "highly skilled" workers' jobs.
Unite Regional Officer Davy Thompson said: "Today we are witnessing yet again a culling of working class jobs in the Ballymena area which will impact across the whole of Northern Ireland, given the supply chain."
He added: "This is the last British company to build double deckers. If Boris Johnston wants to do what he says he wants to do in terms of British industry, following October 31, then let's do it today.
"Let's start standing up for British industry within the global market and let's start putting the cause of British industry first".