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 the I.R.S. will struggle to promptly answer telephone calls from taxpayers with questions and that a lower level of service should be expected. They blamed the lack of resources on Republican legislators, who have blocked efforts to increase funding at the tax collection agency.

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. That request is part of the administration’s proposed spending package, the Build Back Better Act, which is stalled in Congress.

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

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 that 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.