The Federal Government Service Task Force says that proposed budget cuts would cost federal workers and retirees $39.5 billion over the next five years. Friday's column said incorrectly that the projected loss in pay and retirement benefits would occur in three years.

Federal workers and retirees will lose an estimated $39.5 billion in pay and annuity increases over the next three years if Congress sticks with the program begun last year to make major spending cuts come out of pay and pension checks.

A new study by the congressional Federal Government Service Task Force says that cuts already approved--or in the works--have made or will make big inroads in benefits due government workers.

The task force, headed by Rep. Mike Barnes (D-Md.), hopes to persuade House leaders to reject Senate and White House budget plans. It calls for a cap on U.S. pay and pensions, and a partial Social Security tax on workers.

Data worked up by the task force indicate federal retirees have already lost $742 million because Congress, in 1981, changed the law that had allowed them to get twice-yearly cost of living raises to keep pace with inflation, and instead gave them raises only once a year.

And on top of that, the congressional unit says that 8.9 percent of the $443 billion in projected budget cuts over the next three years will come from the wallets and pocket books of government employes. Here's how the cuts would hit:

* Capping federal pay and pension increases at 4 percent would cause white collar feds (average salary here is $26,000) to fall further behind industry, and allow inflation to outdistance retiree income.

Federal workers got a 4.8 percent raise last year when government data showed they were due a 13.46 percent adjustment to bring salaries up to par with industry.

This year, the task force says, the federal pay raise should be 15 percent. It estimates the catch-up raise for 1984 should be over 18 percent.

* The proposal to charge federal and postal workers a 1.3 percent Medicare tax -- ranging from $100 to $450 per year depending on salary -- next year would cut into the take-home pay of workers.

House members who represent big blocs of government workers hope to use the data to persuade their colleagues to back the Post Office-Civil Service Committee and reject the Reagan administration budget.

The House, which was scheduled to take up the federal worker portion of the budget reconciliation package yesterday, now plans to vote on it Tuesday. That will give the pro-fed lobby more time to argue that it's time to stop beating up on the bureaucrats.