HM Revenue & Customs yesterday extended the deadline for people to complete their self assessment tax returns following problems with its website.
The Government department said it was giving people an extra 24 hours to get their returns and any tax they owe to it, without people being liable for a £100 fine.
The move comes after the website crashed at around 10.30am yesterday as thousands of people rushed to get their returns back before midnight.
An HMRC spokesman said: "HMRC's Self Assessment on-line filing service has experienced technical difficulties this morning which has meant that some taxpayers have experienced difficulties filing on-line.
"The system is now rapidly returning to normal levels of service. A record number of 3.6 million taxpayers have already successfully filed on-line with 104,000 filing yesterday.
"HMRC takes any disruption of service very seriously and to reflect this no one who files electronically or by paper by midnight on Friday, February 1, will face a penalty."
Around 150,000 people filed a self-assessment tax return on January 31 last year, and the number of people leaving it to the last day is expected to be similar this year.
Those who miss the deadline incur a £100 charge. Matthew Elliott, chief executive of the TaxPayers' Alliance, accused HMRC of letting down the public.
He said: "This would be laughable if it wasn't so serious. HMRC have yet again let down the taxpaying public, who only want to file their tax returns on time as they are told to. A huge amount of money has been spent on this computer system and on advertising it, so it is appalling that it has failed so badly."