The U.S. Labor Department has accused the U.S. Postal Service of industry-wide violations of fair labor laws in a lawsuit that could result in the award of millions of dollars in back pay to the nation's 600,000 postal workers.

The suit follows by three days the settlement of previous union-sponsored suits against the postal service over such things as hours, pay differentials and overtime. In that settlement, the postal service agreed to set up a fund of $56 million to distribute among the 80,000 individual postal workers named as plaintiffs in those suits.

According to union figures, those individuals could receive about $650 each in back pay as their salaries are readjusted to compensate for past violations.

Yesterday's action by the Labor Department covers the remaining postal workers. Although neither the Labor Department nor the unions said so yesterday, similar settlements for the other postal workers could run into hundreds of millions of dollars.

The Labor Department focued in its complaint on 13 areas in which the Postal Service allegedly has failed to properly pay its workers since May 1, 1974, when the applicable federal labor laws took effect.

Among them:

Failing to pay employes for overtime that had not been previously authorized.

Failing to pay employes for hours they worked learning required postal route systems.

Failing to compensate employes for travel time in connection with training.

Failing to include cost-of-living allowances in the pay of workers who were employed in areas such as Alaska and Hawaii.

A Labor Department lawyer said "it is too early to make an estimate" of the amount of money involved in the complaint, saying that would be determined as the suit develops.

In the previous private suits, which were endorsed by the American Postal Workers Union, the National Association of Letter Carriers and the Mailhandlers Division of the Laborer's International Union, attorneys for the Postal Service agreed to end all pay violations within the next few months and pay the employes for past violations.

The Postal Service's redesigned payroll system should take effect later this year, and will result in the back payments being made.

Union officials said the average payout should be about $650 for employes who worked fulltime between 1974 and now, but could reach about $1,3000 for some employes with large amounts of overtime.

Persons involved in the previous litigation said it could take more than two years for the complex scheme of repayment to be completed for some workers, mostly because of the large number of individual work records that will have to be reviewed.

The Postal Service also agreed to pay all legal fees and expenses arising out of the previous litigation, according to the settlement.

The settlement followed U.S. District Judge June L. Green's finding that violations of the Fair Labor Standards Act had occurred.

One union official said that based on the number of people employed by the Postal Service who might be affected by the new Labor Department suit, the actual payment to postal employes could reach $400 million.

There was no immediate indication how the postal service planned to raise the money necessary to make the back payments.