Hundreds of thousands of Obamacare sign-ups are about to lose coverage if they don’t cough up more information about their citizenship and immigration status.

Obamacare administrator the Centers for Medicare and Medicaid Services announced Tuesday that the agency sent letters notifying 310,000 customers who have failed to fix errors in their citizenship or immigration data that their coverage will be terminated Sept. 30 if they don’t submit proof by Sept. 5.

The customers are part of close to 1 million Obamacare sign-ups who submitted applications with citizenship and immigration information that didn’t square with federal records. CMS claims 450,000 of those cases have been resolved, but it’s not clear how many of those “closed” cases resulted in more canceled policies.

CMS claims it reached out to customers between five and seven times, through mail, phone and e-mail, to try to straighten out the citizenship and immigration errors. They’ve pledged to reach out again with two more phone calls and one more e-mail before Sept. 5.

“We want as many consumers as possible to remain enrolled in Marketplace coverage,” said CMS chief Marilyn Tavenner. “So we are giving these individuals a last chance to submit their documents before their coverage through the Marketplace will end.”

While 1 million out of 8 million sign-ups submitted questionable citizenship and immigration information, another 3 million applications are plagued with problems related to residency, income, incarceration, Indian status, Social Security number and eligibility for another federal health-care program, according to a report Obamacare contractor Serco prepared for House Republicans in June. (RELATED: House Confirms Half Of All Obamacare Applications Are Flawed)

While the administration is working through its hefty backlog of applications with citizenship and immigration flaws, it’s waiting to work on applications with income verification issues. Customers with problems relating to their reported income “will hear from the Marketplace at a later date,” according to CMS. Income-related issues could leave Obamacare customers with an unexpected tax bill if customers are receiving premium subsidies.

CMS is addressing HealthCare.gov applications alone, however. States that are running their own exchanges are on their own to deal with inconsistencies in Obamacare applications. A report from a federal watchdog in July found that a number of states with technically-challenged exchange websites are unable to resolve their flawed applications, including Oregon, Massachusetts, Nevada and Vermont. (RELATED: Feds Have Resolved Less Than 1% Of Flawed Obamacare Applications)

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