"If mobile network operators decline to supply us, we do not have a business," said Phones 4U boss David Kassler.
The company said established mobile contracts taken out through it would not be affected, although phones ordered and not despatched - for example anyone ordering the new iPhone 6 over the weekend - would be.
It 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".
The chain has put 5,596 jobs at risk.
A spokesperson for EE said the decision not to renew its contract with Phones 4U was "driven by developments in the marketplace that have called into question the long term viability of the Phones 4U business".
The spokesperson added that the decision was also "in line with our strategy to focus on growth in our direct channels".