HM Revenue & Customs (HMRC) has relaxed the penalty for some people unable to submit paper tax returns by the new deadline of Friday 31 October.
The £100 fine will not be imposed if forms, delivered by hand, are at Revenue offices before they open on Tuesday, 4 November.
But the HMRC warned taxpayers not to rely on this leeway, as some of its offices do not have letterboxes.
Those returns which are sent by post must still arrive by 31 October.
"People should get their details in well before the 31st," said an HMRC spokesman.
"Some of our offices don't have letterboxes as they are shared sites with other government departments or private organisations," he pointed out.
This year, the self-assessment system is undergoing its biggest change since it was first introduced in 1996, with the forms being significantly revamped.
There is now only one deadline for paper returns, which has been brought forward by three months from the end of January.
Clarification
A widespread advertising campaign, alerting people to this change, currently features the former BBC newsreader Moira Stuart.
"There is general concern that the penny hasn't dropped about the new deadline for many people"
In the past, the Revenue had usually accepted that paper returns filed late would avoid an immediate £100 fine if they were "on the mat" the day after the deadline.
But this is the first time it has explicitly relaxed the penalties for late filing, so far in advance.
"We have always had this to a degree, with a day's grace built in," said John Whiting of the accountants PwC.
"But "on the mat" on Tuesday 4th is taking it publicly a little bit further."
HMRC said there had been no change to the rules and that it was simply clarifying its normal arrangements.
"It is a little more complicated this year because of the closure of our offices at 5.00pm and no opening over the weekend," a spokesman said.
"It is just that we hopefully this year have spelled it out in a much clearer way, so that there can be no doubt as to when a penalty will be charged or when an inquiry can be opened."
The clarity on the matter was welcomed by Chas Roy-Chowdhury of the ACCA accountancy body.
"They have clearly gone the extra mile for taxpayers who get it a bit wrong," he said.
"I also think there is general concern that the penny hasn't dropped about the new deadline for many people," he added.
Online filing
Anyone worried about missing the new deadline can chose instead to file their return online, which gives them much more leeway.
These internet submissions have kept their old deadline of 31 January 2009.
Already 1.7 million people have submitted their income tax details online, 32% more than at the same stage last year.
In the last financial year 3.8 million people used the online method, out of nine million in the self-assessment system.
The Revenue points out that if you incur a fine for filing a paper return late, sending in a return later via your computer will not remove the penalty.
Ian Miles, of accountants James Cowper, warned that some taxpayers, such as those in charge of trusts, cannot take advantage of the online system at all.
"Some returns can be extremely complicated and the Revenue's online filing software simply hasn't coped in the past," he said.
"If your return is particularly complicated you may not realise that you are unable to file online until after the 31 October deadline has passed."
Although this will force some people to make an apparently late return on paper, the Revenue's guidance, recently published on its website, makes it clear that these will not incur a penalty so long as they are in by 31 January.
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