The GMB union said it has been told a further 1,332 jobs are set to go at collapsed retailer Wilko with 52 stores shutting.
There had been hopes more jobs could be saved after B&M struck a deal to buy up to 51 Wilko stores from administrators.
However the union said workers affected by the latest cuts would be informed at the latest by 10am on Wednesday.
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Andy Prendergast, GMB National Secretary, said: "Every single redundancy is a person who will wake up facing an uncertain future. This needs to be on the forefront of everyone’s minds.
"The reality is years of mismanagement have led us here.
"We are still doing everything we can secure a deal that would protect the majority of jobs and stores. But this will be of little comfort for those not knowing how they’ll pay their bills."
Notts-based Wilko, which has around 400 shops and around 12,500 people, went into administration on August 10 after failing to secure a rescue deal.
Stores have remained trading as PwC sought to find a buyer for the business – amid warnings that closures and redundancies would follow.
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Administrators previously said that some 269 roles were going at its head office in Worksop, Notts, as of end of play yesterday (Monday) – while further redundancies across two distribution centres in Worksop and Newport, South Wales were set to be confirmed early this week.
Now the GMB has said 24 stores will close on Tuesday September 12, and a further 28 will be closed two days later – leading to 1,016 redundancies.
The job losses also include 299 at the company’s two warehouses. These workers will be invited to a meeting on Wednesday after which they will be allowed to go home, GMB said.
There will be a further 17 redundancies among the company’s digital team at its support centre.
The GMB said that PwC was still working with a bidder for Wilko “who has made an offer for a significant part of business”.
It added: “Whilst there remains a possibility that this fails to deliver, we are working extremely hard and doing everything we can to help it over the line.”
The union also indicated that workers at the 51 shops that B&M said earlier on Tuesday that it had bought will not transfer automatically to the new owner.
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It said: “Accepting that all details, including even the number of stores involved, are covered by confidentiality agreements, we can say that any such bid would appear to be for store premises only.
“This would mean that whilst new brands may open on site, workers would not transfer to these.”
The union added: “We are making inquiries about the possibility of current staff being given preferential treatment in applying for any jobs that may appear, but at time of writing we cannot confirm these will apply.”