With just hours until the deadline, 1.9m people are yet to submit their tax returns. If you're among them, you could be courting a £100 fine - but there's still time to get your return in.
If you're among them, we explain what you need to do to meet the deadline and avoid a fine.
Around 11.6 million people are required to submit a tax return by HMRC and time is running out to meet the deadline.
You could face a late file penalty even if you don't owe any tax this year.
So if you're required to submit a tax return - either due to your income, employment status or because HMRC asked you to - don't delay in getting started now.
There are only a few hours to go before your return is due, so if you're still waiting on paperwork to confirm figures, it might already be too late.
Rather than missing the deadline, you can estimate the figures you believe are correct and submit your return as is.
When you receive the correct figures, you can then submit an amendment via the HMRC portal or your tax software.
HMRC can fine you if you're careless in providing wrong information or deliberately mislead it. So make sure your estimate is reasonable and correct the record as soon as you can.
Finding receipts is the most time-consuming aspect of doing your tax return, according to one-third of respondents to a recent Which? survey.
Of course, the best way to cut down on time is to keep your records up to date during the year. But if you're digging out your receipts now, there are still ways to make it easier.
Try to find all your receipts before you sit down to do your return. If you've lost a receipt, you may still be able to claim if you have other evidence, such as bank or credit card statements, or the physical item and a record of where and how you bought it.
Some things can also be claimed at a flat rate - such as mileage costs, working from home, or living at your business premises - so you won't need receipts for every transaction.
Just under half of people in the UK don't know how much they're allowed to earn tax-free, our Which? survey found.
Don't waste precious time puzzling over definitions or filling out the wrong information. Get your head around the rates you'll pay, and any reliefs and allowances that might apply.
You can use our free guides to better understand how your tax works:
Some taxpayers will have already paid most or all of their bill, and you may even be entitled to a refund.
But if your tax comes to more than £1,000, you'll also be asked to pay an instalment towards your 2018-19 bill - generally half of this year's total.
You can't pay on credit card, so you'll need to have enough money to hand.
In cases where it's impossible for you to pay, contact HMRC as soon as possible. They may be able to offer you a payment plan or give you a later deadline if you genuinely don't have the funds.
Keep in mind that phone lines are likely to be busy today, so call as soon as you can.You'll also have to pay interest on any late payments.
You can submit online through the HMRC tool, or through an online tax calculator. The paper tax return deadline has passed, so submitting online is your only option.
The advantage of using a tax calculator is that you'll be given helpful tips along the way and told what information to enter and how it affects your total bill.