Kraft Heinz has "re-evaluated" the future of its Wigan site after a £140m investment was diverted to factories in Europe.
The giant said it was "disappointing" that the Kitt Green site missed out on the funding but was a "direct result" of a ballot of staff last year which rejected a package of measures to modernised working practices at the site.
The company added that it is "working to ensure that the right level of investments continue to be made at Kitt Green which remains one of the most important facilities within our network".
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The £140m funding has instead been handed to the Alfaro and Pudliszki factories based in Spain and Poland respectively.
However Unite said it is seeking "cast-iron reassurances" from Kraft Heinz about the future of the Kitt Green plant.
A Kraft Heinz said: "The proposals for Kitt Green had been fully recommended by all Unite stewards on site, including at national level given the significance of the investment, and the creation of jobs.
"As a direct result of the ballot among staff last year which rejected the package of measures and progress towards modernised working practices that was needed to underpin this investment at Kitt Green, we have re-evaluated the location to build additional production capacity for sauces.
"A decision has been taken for the current sauces investment to be made in our Alfaro and Pudliszki factories based in Spain and Poland respectively.
"Whilst we can congratulate our colleagues in Alfaro and Pudliszki for this significant investment, it is disappointing that on this occasion the Kitt Green site was unsuccessful in securing this opportunity.
"With this decision now made, we will continue with our focus and dedication in driving the improvements needed for us to continue our journey at Kitt Green to be exceptional in everything we do.
"With respect to our current and future investment plans at Kitt Green, we are working to ensure that the right level of investments continue to be made at Kitt Green which remains one of the most important facilities within our network.
"As an ongoing process, the company will continue to evaluate the future growth across our various product categories and capacity gaps.
"Therefore, it remains important that Kitt Green is a possible option in the future when the need arises."
The Kitt Green plant makes Heinz tomato ketchup and employs more than 1,000 workers.
Unite national officer for the food and drink sector Joe Clarke said: "This is a disgraceful betrayal of this loyal workforce.
"They were told to make sacrifices to receive this investment yet it has gone to Poland and Spain without any such demands being made.
"Our members are reeling from this news so we are seeking immediate, cast-iron safeguards that the UK plant and jobs are secure.
"We will not allow the UK workforce to be treated like this and Unite will fully support our members in any next steps that they wish to take.
"The millions of UK consumers, who purchase Heinz products, will be disturbed to learn that the workers who make their favourite products are being treated so badly by Kraft Heinz.
"We will now instigate discussions with senior management at the company as to the future plans and investment for the Kitt Green site. The job security of the workforce is our priority."
