WASHINGTON — Tens of thousands of people with modest incomes are at risk of losing health insurance subsidies in January because they did not file income tax returns, federal officials and consumer advocates say.

Under federal rules, anyone who receives an insurance subsidy must file a tax return to verify that the person was eligible and received the proper amount of financial assistance based on household income.

When the federal insurance marketplace opens for the third enrollment season next Sunday, users will see a new question: “Did your household file a 2014 tax return and reconcile any premium tax credit you used?”

If the answer to that question is no, consumers risk losing the subsidies they receive to help pay premiums. Without such assistance, many would find insurance unaffordable.