Thousands of federal employes who work a four-day week will have to return to the Monday through Friday grind next month unless Congress breathes new life into the "flexitime" program.

About a fifth of the 350,000 civil servants here are on some kind of flexitime, ranging from set-your-own hours schedules to shifts that allow them to work four 10-hour days (without getting overtime pay) as a trade-off for longer weekends.

The House has passed legislation that would make the six-year-old flexitime experiment, which is due to expire at the end of this month, a permanent program.

One of the things flexitime does is exempt the government from the law requiring overtime pay after eight hours a day, which is critical to the 4-day week schedules.

But the flexitime bill is stalled in the Senate.

Sen. Orrin Hatch (R-Utah) is insisting that the flexitime bill include an amendment that would allow nonfederal employers to pay subminimum wages to teen-agers in some instances.

Backers of the Hatch plan, including the White House, say the subminimum wage proposal would create thousands of jobs for a segment of the population that has one of the highest unemployment rates.

Labor union leaders violently oppose the idea.

They believe that the sub-minimum wage would price many older workers out of their jobs, and give companies an excuse to underpay younger employes.

If Congress fails to extend the flexitime program, most agencies can -- and probably will -- continue to let employes work flexible shifts of eight hours.

That form of flexitime allows people to come in earlier or later than normal starting time, provided they put in an eight-hour day.

What would be eliminated would be those 10-hour daily shifts, and programs that allow employes to work more than eight hours a day and get leave "credit" to be used at another time.

To keep employes on four-day weeks, agencies would have to pay them overtime, something they won't do.

The problem is that only the people who work flexitime seem to care very much about keeping it.

Labor unions don't like the overtime waiver, and would rather see the government reduce its standard 40-hour week.

Many managers don't like flexitime because the program must be monitored carefully if it is done right.

There is still a good chance that Congress may extend the flexitime program, even on a temporary basis. But the program doesn't have a very strong lobby, and time is running out.