About 12,000 Asda workers are currently facing layoffs next month for refusing to sign controversial new contracts, a source tells Sky News.
It is understood that all employees who do not sign the contract by November 2 will be fired unless the supermarket chain retreats.
Workers who rejected the contract are on notice, but may still change their minds.
Asda said on Wednesday that the "vast majority" of more than 100,000 people affected by the new contract had signed on the dotted line.
The retailer has not yet responded to Sky News's request to comment on the scale of the crisis.
Asda staff traveled to Leeds across the country to organize their protest march. Pic: GMB
The issue came up early Wednesday when a protest march involving hundreds of workers delivered a petition, containing 23,000 signatures on one of Asda's shopping carts, to the company's headquarters in Leeds.
The march was organized by the GMB union, which opposed the deal for several reasons, including missed breaks and forced labor on bank holidays.
National officer Gary Carter said: "Tens of thousands of members of the public are supporting our Asda workers in their fight against these draconian contracts.
"Contract 6 is shocking – the changes are forcing our members to choose between caring for their families or being able to support them. It's a shame.
"Today we are back at the birthplace of Asda – a proud, family-owned business – asking employers to respect workers and offer them a better deal."
Asda says employees who did not support the changes are meeting a 12-week notice period but may still remain in office by agreeing to the new contract.
Roger Burnley, the chain's chief executive, was urged to step down by the union
He argues that 95% of employees will have better results as a result of the updated contract – with complementary payments until 2021 for those who lose.
He said no one would be forced to work on Christmas or New Year bank holidays and that these shifts would continue to be paid twice by volunteers.
An Asda spokesman said: "While we respect GMB's right to express their views, GMB's Gary Carter's comments this morning about Asda colleagues being dismissed for out-of-work appointments are not true.
"This change represents an investment of over £ 80 million and an increase in real salary for over 100,000 of our hourly colleagues.
"Throughout the consultation process on these changes, we take into consideration the concerns and feedback from colleagues – including Mr. Carter – and make it clear that we understand that our colleagues have responsibilities outside of work and will always help them balance them with the work life.
"We are clear that we do not want any of our colleagues to leave us, and while the vast majority of colleagues have chosen to sign the new contract, we continue to have conversations with those who chose not to try to understand their concerns.
"It is vital that we make sure that we are prepared to adapt to the demands of our market. Change is never easy, but we are determined that Asda remains a sustainable business for its customers and colleagues – now and in the future."
The UK supermarket sector has been engaged in a fierce price war since the financial crisis – a conflict that intensified as the Lidl and Aldi stores grew rapidly and gained market share from the so-called "Big Four."
This struggle came at a price – pushing margins and thus profits.
It forced companies like Tesco, Sainsbury's, Morrisons and Asda to reorganize their store chains and cut jobs to focus on price and customer service.
Asda made substantial progress in recovering lost market share last year, although the latest industry figures provided by Kantar Worldpanel show that network sales fell 0.9% in the 12 weeks to October 6.
This reduced its market share to 15%.
Its planned $ 13 billion merger with Sainsbury's – blocked by competition by the Competition and Markets Authority earlier this year – would have created the UK's largest supermarket retailer ahead of Tesco in terms of market share. .