Retailer Phones 4U, which has an outlet in Penny Street, Lancaster, has gone into administration, putting 5,596 jobs at risk.
The chain, which is owned by private equity firm BC Partners, said its 550 stores would all be closed from today, Monday.
Phones 4U blamed the move on mobile network EE’s decision not to renew its contract, which followed a similar move from Vodafone earlier in September.
Phones 4U boss David Kassler said: “If mobile network operators decline to supply us, we do not have a business.”
The company said established mobile contracts taken out through it would not be affected, although phones ordered and not despatched – such as anyone ordering the new iPhone 6 over the weekend – would be.
Orders of phones yet to be dispatched will be cancelled and refunds automatically paid to customers.
The firm also said staff should turn up to work as normal, when they would be given further details and would “continue to be paid until further notice”.
Networks will continue to provide mobile services to existing customers, so customers will be able to continue using and paying for their phone as normal.
Any existing discount deals will still be honoured, as will phone insurance policies.
Phones 4U customer services remain operational and available on 0844 871 2253, the company adds.
Customers whose phones are being repaired can call 0844 871 2269.
Their repairs will be completed with the phone being sent back to their home address rather than a store.