Almost 4 million people yet to file UK tax return due on 31 January

A late tax return will incur an immediate £100 fixed penalty, even if there is no tax to pay. Photograph: Ascannio/Alamy

Almost 4 million people have yet to file their self-assessment tax return, even though the deadline is just eight days away – prompting HM Revenue and Customs to issue an “act now” plea designed to chivvy people into action.

Those who miss the deadline potentially face penalties including late payment interest at a rate of 7.75% – the highest for 16 years – which means HMRC could be in line for “a windfall” this year, accountants have said.