Weetabix has been accused of "bullying" their workers and threatening to cut their pay in what has been described as a "fire and rehire attack".
Engineers at its Kettering and Corby factories are currently on strike against company moves 'to attack' their wages and terms and conditions.
Some employees have been threatened with the sack, according to reports.
Labour MP Barry Gardiner, who tried to change the law to curb fire and rehire tactics, described the cereal firm's actions as "bullying plain and simple".
Unite General Secretary Sharon Graham said evidence that Weetabix was using 'fire and rehire' tactics was now in "black and white".
Earlier this week, a senior Weetabix director said the ongoing dispute with engineers at the firm had nothing to do with "fire and rehire".
But in a letter to an engineer, the company set out their plans to terminate their employee's current contract and put him on a new contract.
The letter, seen by the Mirror, which was sent in May, said: "You have indicated that you do not feel able to agree to the change [to your terms and conditions of employment] and therefore we are continuing the formal consultation process with you."
"One of the options available to us, if your agreement to the proposed change cannot be reached, is to terminate your current contract of employment and offer re-engagement on a new contract.
"We do hope this won't be necessary and that we are able to reach agreement on this proposed change."
Stuart Branch, Group People and IT Director at Weetabix Food Company, said Unite was aiming to "tie a dispute over Weetabix team's compensation to their wider agenda".
He added: "They have used a selective quote from a past communication that does not reflect the truth of what is being said to our team today.
"They, alongside their union representatives, have been repeatedly reassured that no individual is at risk of dismissal and reengagement. This is not an option being considered.
"We have made it clear that roles exist for all our employees thanks to the ongoing investment being made in our UK factories."
Mr Gardiner told the Mirror: "This is bullying plain and simple from a company that made an £82million profit last year.
"Threatening to fire these loyal workers is not out of necessity - it is out of sheer greed."
A spokesperson for Weetabix Food Company said: "For 90 years we've maintained a strong and productive relationship with our workforce across Northamptonshire to create a world-leading cereal manufacturing capability.
"We have repeatedly reassured our engineering team and their union representatives that no individual is at risk of dismissal and reengagement, and that roles exist for all thanks to our ongoing investment in our UK factories."
Ms Graham said: "Weetabix this week declared ‘fire and rehire’ strategies have nothing to do with the current dispute when they are slap bang in the heart of the new contract letters originally sent to their own engineers.
"It seems Weetabix managers are playing fast and loose with the facts”.
It comes after the Government appeared to side with bad bosses by blocking reforms to stop the use of fire and rehire tactics.
At the time, Mr Gardiner accused the Tories of “filibustering” and being “cowardly” as it blocked his Employment and Trade Union Rights (Dismissal and Re-engagement) Bill.
He told the Mirror: "The Government says they believe these fire and rehire tactics are wrong, but they refused to legislate to stop it. That just shows what liars they are."
Earlier in the week, Weetabix said: “We have repeatedly reassured our engineering team and their union representatives that no individual is at risk of dismissal".
It comes after the arbitration service (ACAS) issued findings on ‘fire and rehire’ tactics.
They warned employers: “Our new advice is clear that fire and rehire is an extreme step that can seriously damage working relations and has significant legal risks for organisations”.
Unite’s Ms Graham urged Weetabix management to get back round the negotiating table to sort the dispute.
