The HMRC self assessment website was down for at least four hours on the day forms had to be completed by and the deadline has been extended.
Self-employed people had to submit their completed forms online before the end of today, but the system was down from about 11am this morning until 3pm this afternoon.
An HMRC statement said the system "has experienced some technical difficulties this morning which has meant that some tax payers have experienced difficulties filing online."
It said its has received a record £3.6 million online forms with 104,00 filed before lunchtime today.
The department will now extend the deadline for people to submit the completed forms to midnight tomorrow.