Randall Parker Foods of Llanidloes deals with a million lambs each year with half going to customers in the EU.
But its general manager, Dale Williams, has said that the company could lose a third of its business.
He has warned that there could be redundancies among the 150 employees, some of whom are already on furlough.
The company says it now has to deal with a mountain of paperwork just to send one vehicle to Europe.
"We are operating for virtually nothing. We're doing it for the sake of keeping the doors open at the moment," Mr Williams said.
Plaid Cymru's Senedd candidate, and Powys County Councillor Elwyn Vaughan said the news was a massive blow to the area.
He said: “This news is of huge concern to all and is a direct result of the Conservative’s Brexit deal which will undermine our red meat sector as it’s already doing to the fishing sector. It’s a disgrace that both our Powys Conservative MPs voted like sheep following the party line rather than stand up for our farming community.
"Lamb production is hugely important in Mid Wales and it’s imperative that Parker Randell the operators of the slaughterhouse are supported by Government and not left at the whim of Westminster Government."