The Reagan administration threw its weight last week behind the legal battle being waged in the Supreme Court by women who charge they are discriminated against illegally under many pension plans.
The importance of the issue goes far beyond a current case at Long Island University in New York. The contested plan there is similar to programs at 3,400 colleges nationally, with 650,000 employees. Similar plans cover employees of many state governments. The decision by the Supreme Court could affect millions of workers and billions of dollars in pension money.
The test case involves a Long Island professor, Diana Spirt, who is challenging a retirement program that pays women 11.3 percent less than men each month because women have a longer life expectancy. Pensions are based on actuarial tables, which show the different life expectancy of men and women as groups.
Insurance and pension plan executives are worried. J. Michael Low, Arizona director of insurance who has another case before the high court, says a loss could mean ''the whole industry would have to be restructured.''
Supporting Diana Spirt and others challenging the present pension system, US Solicitor General Rex E. Lee told the Supreme Court Jan. 11 that the 1964 Civil Rights Act prohibits the common practice of varying monthly pension payments on a basis of the different life expectancies of men and women.
The Supreme Court held in 1978 that women cannot be required to contribute more to retirement plans because tables project that they will live longer and collect more in pension payments.
Mr. Lee said in a brief submitted to the court that the principle of law in the 1978 case is precedent for the current litigation.
''Whether a woman contributes a greater amount of her compensation than a man for an equal benefit or contributes an equal amount for a lesser benefit, the use of sex-based actuarial tables in calculating periodic benefits results in the same discrimination,'' Mr. Lee said.
Pension plan administrators contend that the use of actuarial tables equalizes payments to be received by men and women: the latter may be paid less on a monthly basis but the women's total payments on a group basis will be about the same as those for men. Unless there is a differential, the administrators say, women would fare better than men over their longer lifetimes.
In addition to the potential impact on retirement plans, a high court decision for the women could affect insurance programs where differentials exist because of the use of actuarial tables in fixing charges and benefits. Because of possible effects in other fields, the court may narrow its decision by closely and specifically defining the relative rights of groups and individuals.