Britain’s largest union Unite will leave no stone unturned in the fight to save jobs at 2 Sisters.
Nine hundred jobs are at risk at three poultry plants which are facing closure.
2 Sisters Food Group (2SFG) said it is consulting on closure proposals at three "loss-making" sites.
The sites facing closure are Cambuslang in Scotland, with 450 jobs at risk at Smethwick and Wolverhampton in England, where 350 and 100 posts are at risk respectively.
However the company said that it will be creating up to 1,000 new jobs within the poultry supply chain.
Unite said it would now enter consultation with the company and press the case for investment and alternatives to the planned closures in a series of meetings with the company over the coming days and weeks.
Commenting Unite national officer for food Joe Clarke said: “These plans have caused a lot of uncertainty and dismay for the workers involved.
"Unite will be doing everything it can to support our members and secure their futures following this announcement.
“These plans will impact on communities and employment opportunities in Scotland and the West Midlands.
"Over the coming days and weeks, Unite will be pressing the case for jobs and investment to save the three sites from closure.
“It may not be a viable option for workers at the Cambuslang site to be redeployed because of the distances involved to the other sites run by the company in Scotland, while workers in the West Midlands who keep their job face disruption and uncertainty.
“Workers who have stuck with the company through thick and thin and worked hard to make it a success will inevitably feel a sense of betrayal.
"The 2 Sisters Food Group needs to engage in the consultation constructively and be willing to listen to alternatives to site closures.”
A 2SFG spokesman said: "As we continue to build a better business, we are simplifying how we work and investing in the areas that can make the most positive difference to our UK poultry operations.
"This means that we will be creating up to 1,000 new roles within the poultry supply chain.
"However, we do have areas of significant challenge and regrettably 900 roles will be at risk at three loss-making sites which we propose to close.
"Our focus now will be to support all affected colleagues and explore all alternative options available to us before making any final decisions."
2SFG said it is creating new jobs at three locations in England - Willand, Devon (300 new roles); Site D, West Bromwich (potentially 400 new roles) and Scunthorpe (30 new roles).
It also plans to create 250 new roles in Coupar Angus, Scotland and 35 at Llangefni in Wales.
The Unite union said it would fight to stop the closure of the plant in Cambuslang.
Scot Walker, trade union convenor at the Cambuslang plant, has worked there for almost 30 years.
He said: "Obviously we are dismayed at the news of the possible closure. But it is far too early to accept that this is the final word.
"Unite will do everything in negotiation with the company to find a way to safeguard the future of the site and protect the jobs. What's at stake are more than 450 jobs and the future of the local community."
He added: "The company are saying that staff can be redeployed to other sites run by the company - like Coupar Angus. That is not a viable alternative which dictates the necessity of fighting to save the plant in Cambuslang."