Update, 5:00 PM EDT: The IRS has restored service to its e-file system this afternoon. The IRS has given taxpayers a one-day extension for filing, as the agency's free e-payment system was affected by the outage, which was blamed on a "hardware issue." Our original story continues below.

At midday on the busiest day of the year for the Internal Revenue Service's electronic tax-filing infrastructure, the Modernized eFile (MeF) system has gone offline. Acting IRS commissioner David Kautter informed members of the House Committee for Oversight and Government Reform during a hearing at 10:00am ET that a number of systems associated with tax filings were down and that the IRS is working to resolve the issue. As of 2:15pm, a status message on the IRS' MeF site still read, "The MeF System is currently down. We are working this as a priority."

An IRS spokesperson told Ars via email, "Currently, certain IRS systems are experiencing technical difficulties. Taxpayers should continue filing their tax returns as they normally would."

Individual taxpayers don't interact directly with the MeF system. MeF is only accessible through approved e-filing services, including those offered by software providers such as Intuit and H&R Block. So the MeF outage won't immediately affect taxpayers' submission of returns or result in penalties for late filing. But it will delay acceptance of the returns and could cause problems for the software providers and other authorized e-file providers. There's no word on what the underlying cause of the outages are.

MeF was initially launched 10 years ago for business returns and was scaled up to handle all tax return forms in 2012 after a few years of limited support for individual filings. Originally built on Solaris and Sun SPARC hardware, MeF was one of the most ambitious development projects ever at the IRS, involving the building of a separate data center for conducting software performance testing prior to release. Software is built and load-tested in an offline data center before getting "crossed over" to active in December once support for the new version of forms is finalized.

But things got off to a late start this year; the IRS didn't begin accepting tax filings until January 29 because it had to assess the impact of new tax legislation and perform security and performance checks on a number of key systems before taking MeF live. About 155 million individual tax returns are expected to be filed this year, with a large percentage coming at the very last moment.