Sainsbury's has enraged unions and MPs by refusing to back down in a row over staff pay.
The supermarket giant, which is attempting a £12 billion merger with Asda, has been accused by more than 100 MPs of using an increase in its basic pay as a "smokescreen" for the erosion of workers' benefits such as paid breaks and premium wages for Sunday working.
Sainsbury's attempted to quell the backlash yesterday by saying talks with staff were finished, and it would invest an extra £10m in its proposed rates.
The firm wrote to MPs to defend its proposals, but Labour's Siobhain McDonagh said the plans would leave 9,000 workers worse off.
"How can a company that made a pre-tax profit of £589m last year, with a CEO that receives £930,000 before bonuses, think it right to force a pay cut on thousands of long-standing and loyal members of staff?" Ms McDonagh asked.
Unite, the union representing shop staff, accused the supermarket of "breaking its promises" because it would not allow employees to sign up to the new pay structure voluntarily.
The union, which is seeking legal advice, said the supermarket had failed to consult staff in a meaningful fashion.
"Bosses have cut short the consultation extension over the plans, which will leave thousands out of pocket while still holding a gun to workers' heads with the threat of 'sign up to the new contract or be out of a job'," said Joe Clarke, a Unite officer.
Sainsbury's workers will be moved on to the new contracts in September, and the business will give top-up payments to those negatively affected for 18 months.
The supermarket will review pay again in March 2020.
A Sainsbury's spokeswoman said there was "no legal merit" to Unite's claim.
She added: "We are disappointed with this reaction from Unite because they have had a seat at the table throughout the consultation process.
"We ran the consultation period until its natural conclusion because there were no counter-proposals on the table that we had not already considered and responded to.
"We informed Unite yesterday that we had no grounds to continue consultation.
"We have conducted meaningful consultation with colleagues and have invested a further £10m on top of the original £100m to make changes in response to their feedback."