Self-assessment taxpayers have until midnight to file their returns and pay, or they will face hefty fines.
The Inland Revenue is urging people not to panic
Nearly 900,000 taxpayers missed the 31 January deadline last year and received an automatic £100 fine as a result.
However, large numbers of people have told the BBC that they are having difficulties using the Inland Revenue's online self-assessment filing system.
But the Inland Revenue downplayed the problem, adding that people missing the deadline will have to pay a £100 fine.
WHO COMPLETES TAX RETURNS?*
Name or member of Lloyd's
Ministers of Religion (of any faith or denomination)
*Full list and exceptions on Revenue website
"Any difficulties that people are having filing online is purely due to high levels of internet traffic, they just need to keep on trying," said Patrick O'Brien, a Revenue spokesman.
"It is worth bearing in mind that people have had since April to complete their self assessment forms."
In addition to the £100 automatic fine, people who do not pay their tax bill by 31 January will also be charged interest at 7.5% on money owed.
The Inland Revenue will also impose a 5% levy if the debt has not been paid by 28 February, and another 5% on any money still owed on 31 July.
An estimated 9.5 million people have received returns for tax year 2003/4.
By the end of December, the Revenue had received six million completed returns.
"If you still haven't sent the form by Monday, deliver it by hand to your local office," said John Whiting, tax partner at Pricewaterhouse Coopers.
People who are waiting for information to complete their return should offer estimates rather than delay sending it in, said Mr Whiting.
"If you use an estimated figure, declare on the form that it's an estimate, and highlight that your total tax bill is estimated."
Many local Inland Revenue enquiry centres will remain open until 8pm on Monday, so that people can hand deliver last-minute returns.
"If you need help, call our helpline on 0845 9000 444 or your local tax office and, finally, please do not forget to sign it and pay what you owe," Mr O'Brien said.
Have you encountered problems completing your self assessment form online? Your comments.
I completed my self assessment forms online last July - it was working perfectly well then. People shouldn't leave it to the last possible day to submit their returns online.
Alan, Stevenage, UK
I completed my tax return online on 29th, but the system could not confirm it had been received. It had still not been confirmed on Sunday. Today, Monday 11am, I called the helpdesk and they confirmed there was a fault on the system but to keep on checking
Richard Starlig, Hitchin, Herts
Tried many times over the weekend and couldn't connect. Called the help line this morning and they admit to the system being overwhelmed - however £100 penalties WILL be sent automatically regardless
Paul Gillman, Essex
I filed mine 23/1 and it was working perfectly. I nearly left it until this weekend - I'm so glad I didn't!
Donna Cornish, Northwood, Middx
This online fiasco sums up the Inland Revenue perfectly. They've been recommending that people use the online service for months, but when it comes down to it, they haven't the capacity to cope.
Mark, Northwich, Cheshire
If the revenue declare a deadline of 31st of January and their systems don't work then this is clearly their problem and the public should not be penalised. As usual the revenue rides roughshod over the public it's supposed to be serving with no apparent accountability.
Gwyn Carwardine, Hadlow Down, East Sussex
Completed my assessment on Saturday evening, no problems then. Needed to confirm one detail before sending, so tried to submit this morning and found it very hard to connect until about 11.15am when suddenly everything flew through.
Sarah Pencavel, Lancashire
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