Taxpayers are going to have a harder time getting their tax returns prepared this year, and many items that were approved in the past no longer will pass muster.

Under recently passed legislation, professional return preparers, including accountants and lawyers, are now confronted with more severe penalties and tighter controls on what they can and cannot do.Previously, a professional preparer did not hesitate to approve a tax return position so long as he had a "reasonable basis" for his decision. In the eyes of some, this justified almost any apparently plausible claim, even where the chance of success on challenge was as low as 10 to 15 percent.

"So long as you don't chuckle out loud," the cynics said, "it's all right to write it off."

Today this "reasonable basis" rule no longer is acceptable. It has been replaced by a tougher "realistic-possibility-of-success" standard, carrying a $250 penalty (formerly $100) for each violation. Under the new Improved Penalty Administration and Compliance Tax Act (IMPACT), the return preparer is exposed to liability if:

- Any part of an understatement of income tax is attributable to a position for which there is not a realistic possibility of its being sustained on its merits.

- The preparer knows, or reasonably should have known, of the position.

- The position is not adequately disclosed or is frivolous.

If the preparer acts "willfully," "recklessly" or "intentionally," the penalty increases to $1,000 (formerly $500).

Moreover, if a return preparer violates the new law, he risks not only monetary penalties but also his right to practice before the Internal Revenue Service.

How high a standard, then, must a preparer meet in order to support a tax return position today?

Congress stated that it was basing the new rule on the professional conduct standards officially adopted by lawyers and certified public accountants. Yet neither of these professions has proposed a specific estimated chance of winning as the point for taking a tax return position favoring the taxpayer.

A special ABA task force did suggest that a likelihood of success "approaching one-third" was a good rule of thumb to follow.

But regardless of the precise numeration, getting a professional preparer's acceptance of an aggressive tax return position will certainly be more difficult to achieve this year than ever before.

Taxpayers, too, must grapple with their own set of penalties for tax return inaccuracies - for example, for negligence or disregard of rules or regulations, for substantial understatements of income tax, and for substantial valuation overstatements and understatements.

Generally, a single penalty of 20 percent is imposed for an underpayment due to one or more of these errors.

Still in effect is the penalty on taxpayers for a "substantial understatement of income tax." Often criticized as a "no fault" penalty, it applies almost automatically if the amount of the individual's understatement exceeds $5,000 or 10 percent of the tax required to be shown on the return - whichever is greater.

But except for tax shelters, relief is at hand if the taxpayer can show there is (or was) "substantial authority" for the position he took, or if he adequately discloses the relevant facts.

Full disclosure goes to the heart of the new penalties and is encouraged throughout. By specifically describing the transaction in question in the tax return itself or in an attached statement, a taxpayer can be freed from most inaccuracy penalties.

At the same time, taxpayers realize that "flagging" these controversial items is tantamount to extending an open invitation to an IRS revenue agent to begin a full examination of the return. And few welcome such a visitation.

Suggestions by taxpayer-clients and self-protective reactions by preparers may spark more controversy between them and modify the tenor of their relationship.

In brief, taxpayers will find preparers scrutinizing issues more carefully and raising more questions on what and how to report tax items in this year's returns. The previously easy response embodied in the familiar phrase, "My accountant said it was all right," no longer is so easy.