WASHINGTON — About 800,000 taxpayers who enrolled in insurance policies through HealthCare.gov received erroneous tax information from the government and were urged on Friday to hold off on filing tax returns until the error could be corrected.

The Obama administration, under heavy pressure from congressional Democrats, also announced that it would give several million people more time to buy health insurance so they could comply with federal law and avoid tax penalties.

The tax mistake, affecting taxpayers in 37 states, is the first major problem to surface in an otherwise smooth second enrollment period for the Affordable Care Act. The online insurance marketplaces have exceeded enrollment targets, and insurance premiums have generally come in lower than expected. Nonetheless, the mistake could cause some hardship for thousands of lower-income Americans who qualified for subsidized insurance, had hoped for tax refunds and now must wait for weeks to file their taxes.

“Some consumers will be very frustrated,” said Christine Speidel, a tax lawyer at Vermont Legal Aid, “because they count on those refunds to buy home heating oil, to pay for car repairs and to pay off credit card bills.”