Fashion chains Peacocks and Jaeger have fallen into administration, putting more than 4,700 jobs and almost 500 shops at risk.
It comes after owner Edinburgh Woollen Mill Group failed to find a buyer for both businesses.
No redundancies have been announced yet and no stores closed.
EWM Group blamed the pandemic for a collapse in trade, but said it was still in talks with potential buyers.
"In recent weeks we have had constructive discussions with a number of potential buyers for Peacocks and Jaeger," it said in a statement.
"But the continuing deterioration of the retail sector due to the impact of the pandemic and second lockdown have made this process longer and more complex than we would have hoped."
'Desperately difficult decision'
It said that a "standstill agreement" secured with the High Court that temporarily put off administration had now expired.
"Therefore as directors we taken the desperately difficult decision to place Peacocks and Jaeger into administration while those talks continue," it said.
Jaeger is a London-based fashion business with 76 stores and concessions and employs 347 staff. Cardiff-based Peacocks operates 423 stores with 4,369 staff.
Some Peacocks stores had already begun to shut following an announcement on 15 October.
Philip Day has quietly built a retail empire over the last 18 years and his huge footprint on high streets across the UK served him well. He did so by snapping up a string of businesses out of administration.
Critics say he didn't do much with them. He didn't do much business online, either. That left him and his chains more exposed when shoppers stampeded to the internet during lockdown.
Mr Day still has Bonmarche, a business which he bought through a separate investment vehicle and isn't affected by the current insolvency process. It went into administration last year amid challenging trading conditions. He went on to buy it back, with fewer stores.
Will he now do the same with Peacocks or walk away? According to the most recent company accounts, the EWM Group made a pre-tax profit of £31m in the six months until March 2019. It also had plenty of cash in the bank and next to no debt. How quickly things have unravelled.
The news come two weeks after Edinburgh Woollen Mill Group called in administrators for its eponymous clothing chain and its homeware brand Ponden Home, putting almost 3,000 more jobs at risk.
Tony Wright, joint administrator of the business from FRP Advisory, said talks with potential buyers for Jaeger and Peacocks were at an advanced stage, suggesting the brands could still be saved.
"Jaeger and Peacocks are attractive brands that have suffered the well-known challenges that many retailers face at present," he said.
"We are in advanced discussions with a number of parties and working hard to secure a future for both businesses."
Edinburgh Woollen Mill (EWM) is owned by businessman Philip Day, who has a £1.14bn fortune, according to the Sunday Times Rich List published in May 2020.