Employers “derisking” their pension plans no longer would be allowed to give plan participants currently receiving monthly annuity benefits the option to convert their benefits to a cash lump-sum, the Internal Revenue Service disclosed Thursday.

The IRS said in a statement accompanying Notice 2015-49, it will amend current regulations so that pension plans will not be “permitted to replace any joint and survivor, single life, or other annuity currently being paid with a lump sum payment or other accelerated form of distribution.”

The new restriction generally takes effect on July 9. However, the ban will not apply to conversions that already have occurred, as well as in certain situations, including those in which an employer prior to July 9 informed plan participants of such an offer or when an employer formally adopted such a change as part of a collective bargaining agreement that was approved prior to July 9.

Experts say to date, only a small percentage — less than 5% of employers, including Ford Motor Co. — have given plan participants currently receiving benefits the option to convert their monthly annuity to a cash lump sum. Typically, such offers have been made to former employees who have earned an annuity but are too young to start receiving the benefit.

The key reason that such offers have not been extended to retirees collecting benefits is the fear of adverse selection, said Mike Archer, a senior actuarial consultant with Towers Watson & Co. in Philadelphia.

That is because, Mr. Archer says, retirees in poor health would be more likely to take the lump sum than those in good health.