The penalties relate to those who have still failed to fill in forms for the 1997-98 tax year and who have already been fined pounds 100 for missing the last deadline at the end of January.
On top of that, they also face 5 per cent surcharges on any tax that is owed, plus interest payments of 7.5 per cent on the accruing total, backdated to January.
Tax advisers say that people still don't understand the forms, despite publicity blitzes by the Inland Revenue, and are calling for a simpler system.
Figures released yesterday show that 748,000 people were fined for missing the January deadline, only a slight improvement on the year before, which was the first in which self-assessment came into force. The exact figure for the next stage will not be known until after tonight's deadline, but if it also reflects last year, a further pounds 33.5m will be heading into government coffers.
The fines will apply to all, even if people are actually owed money by the Revenue. But a spokeswoman yesterday denied that these were a means of raising extra money.
"Our aim is to make sure that everybody fills these forms in and gets them in on time," she said. "The fines are supposed to act as a deterrent."
An indication that the system was working was the fact that 87.9 per cent of self-assessment forms were filed in time, added the spokeswoman.
Most local tax offices will be open over the weekend for those wanting to make a last-minute submission, and assistance is available from the self-assessment helpline, and a dedicated website.
"We are working very hard at getting the message out that we are here to help," she said.
But research by the independent tax advice firm ie taxguard suggests that about half of taxpayers find the forms baffling. "Something is very wrong with the UK tax system when more than 700,000 consumers are fined for missing tax deadlines when, in truth, many - quite understandably - find tax matters utterly confusing," said Mike Crossley, of the Peterborough- based company. "We believe a new approach to consumer tax matters is required to make self-assessment simpler."
The company has also urged the Government to support a broad-based consumer education programme to help with understanding tax matters.
This weekend also marks the deadline for payment of outstanding tax from 1997-98. Those who failed to pay in January have been hit with a 5 per cent surcharge and will be charged another 5 per cent if they still don't pay.
The second payment of tax accounts for the 1998-99 tax year have also become due, and anyone failing to make those on time will have to pay interest until the account is settled.Reuse content