Metropolitan Life Insurance Co. is accused in a new lawsuit of shortchanging workers by using an outdated formula to calculate pension benefits.

MetLife’s pension plan improperly reduced annuity benefits for retirees who received an alternate annuity benefit, according to a lawsuit filed Dec. 3 in the U.S. District Court for the Southern District of New York.

These retirees are allegedly receiving benefits below what they are entitled to under the federal Employee Retirement Income Security Act.

MetLife is reviewing the lawsuit and intends to defend itself vigorously, a company spokeswoman told Bloomberg Law Dec. 4 via email.

The lawsuit, filed...