Canada Revenue Agency (CRA) says its online system is down.
In a tweet, CRA says it isn't sure when it will be back up and is apologizing for the cut in services Monday morning, saying officials are looking into the "technical issues."
Some Twitter users say the agency's services appear to have been down since Sunday night.
Both the "My Account" and "My Business Account" log-in pages come up with notices that they are unavailable and both are used heavily this time of year for Canadians filing their taxes.
The deadline to pay any taxes owing is April 30.