The deadline for submitting tax returns online is midnight tonight (31 January), but the tax authorities have granted taxpayers who have already registered on the HMRC website but have lost their activation code an extra two weeks reprieve.

HMRC said taxpayers would have to immediately pay the £100 late filing penalty if they miss the 15 February deadline.

Importantly, the deadline for paying any tax due is still 31 January.

While it is not unusual for HMRC to extend the deadline, Chas Roy-Chowdhury, head of taxation at the Association of Chartered Certified Accountants (ACCA), said it has never been extended by so long.

“In the past the deadline has been extended by a couple of days but two weeks is really generous. HMRC is doing the right thing,” he said.

Roy-Chowdhury said the extension will help the 100,000 people who have been brought into the self-assessment net for the first time because of changes to child benefit rules.

Earlier this week the ACCA called on HMRC to push back today’s deadline after it emerged that two million people have yet to file their tax return.

“The circumstances around this year’s deadline are different in that there will be a high number of people who will never have done self-assessment in their lives. They are going to miss the deadline not because they have been putting it off, but because they are newcomers,” Roy-Chowdhury said.

According to an HMRC spokesperson, as of yesterday, 9 million people had their filed returns. HMRC is expecting more than 600,000 people to file their returns today.