The I.R.S. is warning of a messy tax season.

WASHINGTON — The federal tax filing season will run from Jan. 24 to April 18 this year, the Internal Revenue Service said on Monday, warning in its announcement that staffing shortages and paperwork backlogs could make for a messy and frustrating experience for taxpayers.

In a briefing on Monday, Treasury Department officials said that the I.R.S. would struggle to promptly answer telephone calls from taxpayers with questions and that a lower level of service should be expected. They blamed Republican legislators, who have blocked efforts to increase funding at the agency, for the lack of resources.

The Biden administration has asked for an additional $80 billion over a decade for the I.R.S. to bolster its enforcement and its customer service capacity, raising its staff by nearly 87,000 employees and upgrading its technology. That request is part of the administration’s Build Back Better Act, which is stalled in Congress. The Treasury Department estimates that enhancing the enforcement powers of the I.R.S. could yield the federal government $400 billion in additional tax revenue over a decade by shrinking the so-called tax gap, or tax money that is owed the government but goes uncollected.

Treasury officials noted that in the first half of last year, fewer than 15,000 employees were available to handle more than 240 million calls — one person for every 16,000 calls.

Although the population of the United States has grown by about 60 percent since 1970 and the tax code has become more complex, the size of the work force at the I.R.S. has been flat, the Treasury Department said. The agency has fewer auditors now than it has employed at any time since World War II.

“In many areas, we are unable to deliver the amount of service and enforcement that our taxpayers and tax system deserves and needs,” Charles P. Rettig, the I.R.S. commissioner, said in a statement. “This is frustrating for taxpayers, for I.R.S. employees and for me.”

He added: “Additional resources are essential to helping our employees do more in 2022 — and beyond.”

Republicans seized on the Biden administration’s plans to beef up the I.R.S. last year, issuing dire warnings that the agency would be deployed to spy on Americans and target conservatives. Their attacks succeeded in killing a proposal that would have required banks to submit additional taxpayer data to the I.R.S.

The tax season this year will be more complicated than usual because pandemic-related economic impact payments and child tax credit payments were distributed last year. Taxpayers will be required to report the amount of money that they received.

The I.R.S. is urging taxpayers to file their returns electronically, and said people should generally receive refunds within 21 days of filing.

Usually Tax Day falls on April 15, but it will be delayed for most tax payers to April 18 because of the Emancipation Day holiday in Washington. Filing deadlines for state taxes may differ.