The federal government now puts about $10 billion a year into retirement funds for its civilian employees; it spends one dollar on pensions for every four it spends on pay.

The federal retirement system is more generous than almost any in the private sector. In certain unionized industries like autos and steel, retirement benefits from Social Security and company pension plans combined may be about as large as under the federal system.

But most workers in America have no private pensions to add to Social Security.

These are among the reasons why federal employee unions have united in opposition to legislation that will come up in the House this week, to phase out the present federal retirement system, put federal (and state and local government) employees under Social Security, and then presumably create a new federal system to supplement Social Security.

The House Ways and Means Committee proposed the step partly as a way of adding to the income of the ailing Social Security system, at least in the short run; at least at the outset, contributions from and in half of government employees would outstrip the Social Security benefits they were owed, and the system would gain ground.

But the federal employees fear they would lose ground - and consequently there is a good chance the Ways and Means recommendation will be defeated this week on amendments from Reps. Joseph L. Fisher (D-Va.) or Gladys Noon Spellman (D-Md.), both of whom have lots of federal employees in their districts.

A $15,000-a-year federal employee could retire at the end of this year after 30 years of service and receive a retirement benefit of $703 a month. The maximum any Social Security beneficiary can receive is $460 in basic benefits, plus supplement of various sizes for dependents.

The civil service advantage doesn't end there: the federal employee can retire after 30 years of service at age 55. The Social Security beneficiary can't retire until he is 65 if he wants to get his full benefit, although he can opt for age 62 with a 20 per cent benefit reduction.

The civil servant, after retirement at 55, is free to work at any job he wants at any salary - $10,000, $15,000, $20,000 a year - without reduction of benefits. The Social Security beneficiary, on the other hand, would have hs benefits reduced if he earns over $3,000 from any outside job once he begins receiving Social Security benefits.

Most members of the Ways and Means Committee assumed that the way the bill would work would be to phase out the current civil service system and restructure it so that federal employees would have a two-tier system of taxes and benefits, just as in private industry where a worker pays for Social Security but also can become eligible in some firms for a separate company pension on top of his Social Security benefit.

Under this assumption, federal employees would pay for and receive Social Security benefits just like any other workers. On top of that, they would receive a federal pension under the same general age-55 and eligibility requirements as now, to bring them up to the same levels they would have been receiving under the system that exists now.

Their combined contributions to the basic Social Security and supplemental government pension system would be no greater than today. At present, they pay much more into the federal system - 7 per cent of their entire salaries - than workers under Social Security pay in payroll taxes (5.85 percent of the first $16,500 of income).

That is theoretically how the new system would work. Federal employees wouldn't lost anything except in a few minor respects - but the problem is that these precise guarantees and detailsof the proposed new system - and guarantees that workers caught in the transition won't loose - haven't been worked out.

And that is why federal employees are upset.

"Workers are afraid of the unknown," said Lou Pellerzi of the American Federation of Government Employees. He said some think that when the public looks at the size of federal retiree benefits, it will say push those bureaucrats down to the same levels as Social Security.

Some fear they will lose credits for work already done, or that instead of the total contribution being the same as now, the Social Security tax will simply be laid on top of what they already pay.

For a federal employee making $10,000, this could mean doubling his $700 contribution.

Others fear the attempt to bring federal workers under Social Security coverage is simply an attempt to raid the government employee funds to shore up Social Security.

Commissioner of Social Security James B. Cardwell and former Commissioner Robert Ball have both stated - and actuaries for civil service agree - that it is technically possible to construct a dual federal system combining Social Security with a supplemental federal pension system with age 55 retirement still alive, without substantial increases in cost or loss of benefits to civil servants - and taking care of all the transitional problems so that no one is severely maltreated.

Pellerzi said the public has some misconceptions about the civil service retirement system. For example, he said, the federal benefits look pretty high and they are really quite good. But they have been deliberately set at a level roughly equivalent to what a worker might get in private industry from a combination of Social Security and a good employer-paid private pension plan.

Statistics bear this out: under the United Auto Workers agreement with General Motors, workers get $11 a month in private retirement benefits for each year of service - $330 for a 30-year man. This, combined with his $350-to-$400-a-month Social Security pension could put him at or above a civil service annuitant with the same working-life income.

But only one of three retired workers receives a private-industry pension.

Ball, comparing the two systems, pointed out that in many respects, Social Security gives a worker far more protection if he is low-income or has worked relatively short periods.

For example: Social Security retires get Medicare but federal retirees don't; they must pay $20 and more a month for continuation of health benefits policies.

Long-term low-income workers can receive higher benefits under Social Security than under federal retirement because Social Security is weighted to help poor people more.

Social Security benefits aren't taxable; federal benefits are.

Perhaps more important, Social Security benefits for a young disabled worker can be paid for a lifetime, with medical care in addition, for a worker with as few as six quarters in employment subject to the Social Security tax.

The federal worker gets nothing until he has been in the federal system five years, and the benefits are far lower for young federal workers. The widow of a worker dying young usually gets higher benefits under Social Security.