WASHINGTON — The Internal Revenue Service plans to delay the start of tax-filing season by a week or two because of the government shutdown, the agency said on Tuesday. But taxpayers will still have to turn in their 2013 returns by April 15 as usual.

“Readying our systems to handle the tax season is an intricate, detailed process, and we must take the time to get it right,” Daniel I. Werfel, the acting I.R.S. commissioner, said in a statement.

“The adjustment to the start of the filing season provides us the necessary time to program, test and validate our systems so that we can provide a smooth filing and refund process for the nation’s taxpayers,” he said. “We want the public and tax professionals to know about the delay well in advance so they can prepare.”

Under the delay, the I.R.S. would start accepting returns between Jan. 28 and Feb. 4, the agency said, and would make a final determination in December on when the filing season would start.