Law firms and wealth managers are warning clients that a “significant” estate planning technique could be lost, and the tax cost of transferring interests in family-controlled entities will “increase substantially,” if proposed regulations issued by the U.S. Treasury Department in August become law.

The long-awaited regulations, issued by Treasury under Section 2704 of the Internal Revenue Code, apply to the valuation of interests in corporations and partnerships for estate, gift and generation-skipping transfer tax purposes.

“It is common for wealthy taxpayers and their advisors to use certain aggressive tax planning tactics to artificially lower the taxable value of their transferred assets,” Mark Mazur, assistant secretary for tax policy at the Treasury Department, said in a blog post in August. “By taking advantage of these tactics, certain taxpayers or their estates owning closely held businesses or other entities can end up paying less than they should in estate or gift taxes.”

Treasury officials contend the proposed regulations “will significantly reduce the ability of these taxpayers and their estates to use such techniques solely for the purpose of lowering their estate and gift taxes.” The IRS has scheduled a public hearing for Dec. 1.

Law firms almost immediately sounded alarms over the proposed rules, urging clients in some cases to make certain transfers now before any rules are finalized.

McDermott Will & Emery advised clients that the new rules, if adopted, “are likely to substantially increase estate taxes payable by the estates of owners of family-controlled businesses, farms, real estate companies and investment companies.”

Lawyers at Pillsbury Winthrop Shaw Pittman told clients that the proposed regulations “limit or eliminate certain discounts currently recognized under the law that are used when valuing certain transfers of an interest in a family-owned or closely held business,” such as corporations, partnerships or LLCs, business arrangements that affluent families generally use to hold a portion of their wealth.

“The scope of the proposed regulations as drafted may potentially have a much broader impact on legitimate valuations,” the Pillsbury attorneys wrote.

Indeed, Steve Akers, senior fiduciary counsel at Bessemer Trust, wrote in a client alert last month that the regulations “may significantly restrict (or eliminate) lack of control discounts in valuing interests in entities and may impact marketability discounts as well.”

Arent Fox noted in an alert that clients who are over the federal estate tax exemption threshold—for 2016, $5.45 million per individual or $10.9 million for married couples—and are considering transferring interests in family-controlled entities “should act quickly to make the transfers as soon as possible, while the planning window is still open.”

McGuireWoods attorneys urged clients who are considering transferring interests in family-controlled entities that are not controlling interests and do not have liquidation rights to consider making the transfers as soon as possible. “It is possible, however, that if the client dies within three years of the transfer and after the date that the proposed regulations become final, the client may be caught by the final regulations,” McGuireWoods lawyers wrote.

Attorneys following the regulations expect a flood of comments from estate planning professionals, with lawyers at McGuireWoods warning that the broad sweep of the proposed regulations will likely bring “challenges to Treasury’s authority to adopt them in their present form.”

So far, 31 comments have been submitted to the Treasury Department. Many of the comments, filed by individuals, expressed concern that regulators acted without congressional consent. Comments are due by Nov. 2.

If finalized in their current form, “most of the restrictions that have historically justified discounts for transfers of interests in family-controlled entities would be disregarded for valuation purposes,” Arent Fox attorneys wrote to clients.

“A significant estate planning technique will be lost,” the Arent Fox lawyers wrote. “and the tax cost of transferring interests in family-controlled entities will increase substantially.”