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.
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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.