Owners of small businesses, consultants and other self-employed workers may be in for a shock: On Thursday, the Internal Revenue Service will begin charging some of them $25 a day if they have not filed a new tax form -- which many have not been told they must file.

"There's a whole lot of people about to be bushwhacked by the federal government," said Andrew S. Lang, an accountant with Raymond E. Lang & Associates in Chevy Chase.

Area accountants, notified through the trade press and IRS publications, have told their clients who have so-called Keogh retirement plans that they must file the form. But the IRS apparently has made little or no effort to tell self-employed business persons who complete their own returns that they must send in the five-page form.

Two years ago, the service issued a press release about the requirement, which goes into effect this year. Since then, according to spokesman Steve Pyrek, two notices in the IRS Bulletin have referred to the form, but there apparently has been no attempt to notify affected taxpayers by mail.

Form 5500-C is required for all individuals who have Keogh plans, which allow self-employed persons to defer taxes on income set aside for their retirement. The form does not involve the payment of additional taxes, it merely seeks information about the plan.

But it is complex and detailed, and could take hours to fill out. Jeffery P. Capron, an accountant with the firm of McQuade and Capron, said that "most" of his clients who are covered by the requirement asked the firm to do the work for them. Lang said only 20 of his 80 affected clients were able or willing to fill out the form themselves.

"It's a little onerous," Capron said. "This form is not easy to do. There are a lot of complicated things on it and it's always harder to do it for the first time."

It is not clear how many people are covered by the new requirement. In 1983, according to IRS statistics, 663,000 taxpayers made payments to a Keogh plan. But that figure includes all types of Keogh plans, and holders of the most elaborate plans have been required to file the form for some time. The 1982 tax law extended that requirement to everyone with a Keogh plan.

Dominic LaPonzina, IRS spokesman for the Maryland and District regions, said there have been relatively few phone calls about the requirement, although people have been picking up the forms at local IRS offices.

Taxpayers who don't have time to file the form by midnight tonight can get an automatic extension by filing Form 5558, which also is available at local IRS offices. But it must also be in the mail by midnight tonight, or the penalty will be charged.

The snafu over the forms comes as the IRS is still behind in paying refunds on 1984 income taxes. According to spokesman Scott Waffle, about 300,000 people nationwide who filed their returns on time still have not received refunds. Fully half of those taxpayers filed their returns through the agency's Philadelphia service center, which handles returns from the District and Maryland, among other states.

As of July 19, the service had paid $19.8 million in interest on individual income-tax refunds that had not been sent by June 1. Taxpayers expecting large interest payments have been disappointed, however; the IRS dropped the rate it pays from 13 percent to 11 percent on July 1, and the interest it pays covers the period from April 15 up to 10 days before the date on the refund check, not to when the refund is mailed.

Both actions are legal, but they have spawned bills in Congress that would require the IRS to be more generous with its interest payments.