Of Britain's big four grocers - market leader Tesco, Sainsbury's, Asda and Morrisons -Walmart owned Asda is the last to implement more flexible working contracts as it seeks productivity improvements in a brutally competitive market.
Asda's new standardised contracts increase the base rate of pay for over 100,000 retail workers to 9 pounds ($11.58) per hour, plus premiums, while maintaining benefits including an annual bonus, share save scheme and staff discount.
However, they remove paid breaks and require employees to work across departments and work on some bank holidays.
Asda has said workers have until Nov. 2 to sign the new contracts when the 12-week notice period of their various current contracts ends. Without a contract they will not have a job.
"We have been clear that we don't want any of our colleagues to leave us and whilst the vast majority of colleagues have chosen to sign the new contract, we continue to have conversations with those who have chosen not to, to try and understand their concerns," said a spokeswoman for Asda.
She declined to provide a figure for those who had refused to sign, adding that a figure of 12,000 cited by Sky News on Wednesday was "entirely speculative".
"There's still some time for colleagues to make their mind up and decide whether they want to move over to the new contract," she said.
The GMB union has called the contracts "draconian" and its campaign against them has been backed by opposition Labour party leader Jeremy Corbyn.
The supermarket group says the new contract represents an investment of more than 80 million pounds and is not a cost-cutting exercise.
All of Britain's big four grocers are continuing to lose market share to German-owned discounters Aldi and Lidl, which are aggressively opening new stores.
In April Sainsbury's 7.3 billion pounds agreed bid for Asda was blocked by Britain's competition regulator. A month later Walmart said it would instead look at an initial public offering for Asda and in July Asda CEO Roger Burnley said this could happen in two to three years.
(Reporting by James Davey; editing by David Evans)