Retailer Phones4U has gone into administration, putting 5,596 jobs at risk.
The chain, owned by private equity firm BC Partners, said all of its 550 stores will be closed today.
The move has been blamed on the mobile network EE's decision not to renew its contract with the firm, which followed a similar move from Vodafone earlier in September.
The firm said EE and Vodafone's decision to leave had come as a "complete shock".
Phones 4U boss David Kassler said: "If mobile network operators decline to supply us, we do not have a business."
The company have reassured that established mobile contracts taken out with them would not be affected.
A spokesman for EE said the decision not to renew its contract with Phones4U was down to the "developments in the marketplace that have called into question the long term viability of the Phones 4U business".
The spokesman added that the decision was in line with their strategy to focus on growth in their direct channels.
Mr Chassler said it was a particularly "sad day for both customers and staff".
He added: "A good company making profits of over £100m, employing thousands of decent people has been forced into administration."
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