The 2014 tax filing season will be delayed for at least two weeks because of the 16-day partial government shutdown, the Internal Revenue Service announced Tuesday.

This means tax refunds will also be delayed.

The agency said it needs more time to program and test its tax filing system, adding that the shutdown had interrupted its work.

“The government closure came during the peak period for preparing IRS systems for the 2014 filing season,” the IRS said in a statement. “Programming, testing and deployment of more than 50 IRS systems is needed to handle processing of nearly 150 million tax returns. Updating these core systems is a complex, year-round process with the majority of the work beginning in the fall of each year.”

“About 90 percent of IRS operations were closed during the shutdown, with some major workstreams closed entirely during this period, putting the IRS nearly three weeks behind its tight timetable for being ready to start the 2014 filing season,” it adds.

The IRS said it needs to ensure that its refund fraud and identity theft detection and prevention systems will be prepared for 2014.

The filing season had been set to start Jan. 21. Acting IRS commissioner Danny Werfel said the agency is working to shorten the delay and will announce the exact start date in December.

“Readying our systems to handle the tax season is an intricate, detailed process, and we must take the time to get it right,” Werfel 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.”

The delay will affect early filers, many of whom rush their returns to the IRS so they can quickly get refunds.

“The IRS is exploring options to shorten the expected delay and will announce a final decision on the start of the 2014 filing season in December,” the IRS said, adding that the agency would “start accepting and processing 2013 individual tax returns no earlier than Jan. 28 and no later than Feb. 4.”

The IRS during the partial government shutdown received roughly 400,000 pieces of correspondence, the agency said. This is in additional to the roughly one million items that it was already processing before the shutdown took affect. The agency is now looking for ways to deal with its past workload as well as ways to deal with pent up demands.

“In the days ahead, we will continue assessing the impact of the shutdown on IRS operations, and we will do everything we can to work through the backlog and pent-up demand,” Werfel said. “We greatly appreciate the patience of taxpayers and the tax professional community during this period.”

The April 15 deadline for filing individual tax returns remains unchanged.

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The Associated Press contributed to this report.