Various emails told subscribers that they had to update their payment details, that their service had ended and they could restore it with another payment, or that Clearwire couldn't process their latest payment. One subscriber said she received more than a dozen such messages, some bearing account numbers other than her own.

The problem was caused by an error in Clearwire's email system and did not affect users' accounts or their broadband service, Clearwire said. It began some time on Friday, according to a company spokesman. Late Friday afternoon, the company was still responding to complaints on Twitter, saying email messages that were still coming had been queued up and would end soon.

"Current and past CLEAR customers may have received one or more inadvertent emails stating that CLEAR could not process a credit/debit card payment and/or that they needed to make a payment in order to reinstate their CLEAR internet service. Nearly all of these emails were sent in error, and we sincerely apologize for any confusion and inconvenience that this may have caused," Clearwire said in a statement.

"Current CLEAR customers in good standing should not be experiencing any interruption in their CLEAR service, and no action is needed on their part. Former customers who received this email can be assured that we have not attempted to charge their credit or debit card. We believe this issue to be resolved," Clearwire said.

Clearwire had about 1.3 million retail subscribers at the end of the second quarter. Most of its 11 million users are wholesale customers who use its WiMax network as part of their Sprint Nextel service. It lost 8,000 retail and 34,000 wholesale subscribers in the quarter.