A total of 350 offices will open today until 4pm in order to accept forms delivered by hand. Some offices will stay open even later in areas where most returns are late.
The Inland Revenue yesterday said it would still accept the forms if they were "on the mat" when staff entered offices on Monday morning, giving many a crucial breathing space to put forms physically through the Revenue's letter box.
Taxpayers who personally deliver forms to Revenue offices on Sunday will still be treated as if they have arrived on Saturday. However, taxpayers who sent their forms by post yesterday are not guaranteed that they will be treated as hitting the deadline.
A Revenue spokesman said: "What we have said all the time is - don't panic. But it is getting a bit late to put it in the post. If it arrives in the post on Monday it is technically too late. But, like every organisation, people need to make allowances for the post."
More than 1.3 million taxpayers are estimated still to be hanging on to forms, incurring an automatic penalty of pounds 100 each and netting the Revenue up to pounds 130m in fines.
Tax agents say hundreds of thousands of taxpayers will receive the pounds 100 automatic penalty even though they do not owe it. They will be forced to pay if they have not sent in tax returns and have just 30 days to appeal.
In a letter to The Independent published today, chartered accountant Gerry Jackson claims the pounds 100 fines will break the Taxes and Management Act 1970 if the taxpayer is owed money by the Revenue or if the fines which exceed what the taxpayer owes.
Many self-employed taxpayers will be owed rebates if they paid too much in January and June last year. Others have used tax-free vehicles such as personal pensions to wipe out tax liabilities. If agents are late with these returns, no fine should be levied, Mr Jackson said.
"Certainly it seems few people are aware of this get-out which will probably apply to a large number of the 2 million or so taxpayers who have been lax in getting in their returns."
Taxpayers must also pay the balance of tax for 1996/97 and any payment on account for 1997/98, by today at the latest. Outstanding tax received after February 28 will receive a 5 per cent surcharge.
Ernst & Young, which has more than 30,000 self-assessed clients, has told policyholders it believes the Revenue will create an amnesty for taxpayers who have missed the 31 January deadline.
Accountants said the Revenue may be pressured to back down on the deadline for some taxpayers because of a software blunder which affected efforts to process forms on time.
Large firms of accountants found that essential elements of the software, supplied by the Inland Revenue, failed to work with their systems, setting back efforts to process forms electronically.
Ernst & Young, the fourth largest accountant in the UK with 6,570 staff, wrote to its clients last year to tell them of the glitch, which had also affected many other accountancy firms. It said the Revenue might be forced to give an amnesty to people who missed the deadline because of the glitch.
Meanwhile accountants have reported 50 per cent leaps in turnover. Gerry Hart, head of Tax Team, the tax agent network, said: "The publicity has brought people out of the woodwork - in some cases people who have been self-employed but haven't been in touch with the Revenue. In those cases there's not much you can do for them.
But he said self-assessment had sharply accelerated the flow of new clients to accountancy firms. "One of our offices got 25 clients in 3 days the other week. It's done wonders for our business."
Letters, page 18