HM Revenue and Customs (HMRC) has extended the self assessment deadline after problems with its online service.
HMRC says 175,000 returns were filed despite the problems
HMRC's website was out of action for several hours on Thursday, the last day taxpayers could submit their return.
Thousands of taxpayers were facing an automatic £100 fine plus interest on any tax due if they missed the original midnight deadline.
HMRC now says no one who files their return before midnight on Friday will be penalised.
"HMRC's self assessment online filing service has experienced technical difficulties this morning [Thursday] which has meant that some tax payers have experienced difficulties filing on-line," said an HMRC spokesman.
"Any one who files electronically or by paper by midnight Friday 1 February will be treated as having filed on time and will not incur a penalty," said an HMRC spokesman.
"In line with normal practice as our offices are not open at weekends any returns and payments in our letterboxes when we open up on Monday will be considered as being on time," she added.
Despite the difficulties, 200,000 people had managed to file their tax return online by midnight on Thursday, 50,000 more than used the service on 31 January last year.
The Revenue said service levels had returned to normal by Thursday evening, and that it very much regretted any inconvenience caused.
But the BBC has continued to receive reports from people who remain unable to log on.
Edward Buxton from Nuneaton was one of the many taxpayers who contacted the BBC on Thursday to say he was unable to access the online system.
He spent the morning trying to file returns on behalf of himself and his wife, starting at 0900 GMT.
"I have been trying on and off since. Once I got a message from HMRC saying that they are aware of the problem, but most of the time the message just says something along the lines of 'the server has no idea what the page is you are trying to access'.
"That doesn't give you great confidence."
Kevin Butler, from Marlow, also contacted the BBC.
"I have tried to log in repeatedly, and get an obscure failure message after a very long wait by the browser as it tries to access an obviously overloaded website," he said.
"It's not a good time for the service to die."
Chris Kirby is an accountant based in Redcar in North Yorkshire, who was trying to file returns on behalf of six clients.
He said problems first arose on Wednesday afternoon, after which the website "pretty much crashed".
He has not managed to get onto the system since. He said that when he first contacted the helpline, they denied anything was wrong.
Eventually staff told him the problem was down to "too many people trying to log on".
But it is not uncommon for clients to find they cannot submit their information until just before the deadline, he said, and he believed HMRC should be able to cope.
"They should have had not just sufficient capacity, but spare capacity to deal with the peaks they should have known they were likely to get today," he said.
"Really, it's just not good enough for a public body," he added.
More than 900,000 taxpayers missed the 31 January deadline last year, although they did not all receive the automatic penalty, as this is only levied if more than £100 tax is owed.
HMRC published adverts this week to remind people of the deadline
To encourage people to submit their returns on time this year, the Revenue launched a last-minute advertising campaign.
This consists of 848 road side hoarding - 28 of which are at London Underground stations - and 443 adverts at selected ATMs across the country.
Chas Roy-Chowdhury, head of tax at the Association of Chartered Certified Accountants (ACCA), said HMRC had no choice but the push back the cut off point.
"If the Revenue's IT systems have crashed, then they need to extend the deadline. They need to be pragmatic."