Some large users of American Telephone & Telegraph Co.'s Wide Area Telecommunications Service (WATS) face rate increases of more than 10 per cen after Aug. 1, under new WATS rates filed yesterday with the Federal Communications Commissions.

AT & T spokesman said it is difficult to measure the average impact of the proposed rate revisions, ordered last year by the FCC. But AT & T said monthly charges for about two-thirds of the 50,000 interstate customers who use WATS would be reduced, that the remaining one-third will have higher rates and that for 7 per cent of its customers, the average monthly bills would rise more than 10 per cent.

Last May, the FCC ruled that AT & T's WATS charges were unlawful, in concluding an investigation that started in 1974, Commissioner Abbot Washburn said last year, in a statement accompanying the WAS decision that his agency had "essentially lost control over the rates Bell charges customers."

In essence, the FCC said AT & T had structured WATS rates to favor certain sectors of its market while not basing charges primarily on actual costs.

Under the proposed rates filed yesterday, in 52 volumes consisting of nearly 16,000 pages AT & T said its charges would be more closely related to costs of serving various types of customers.

Started in 1961 by the Bell System, WATS lines are used widely in America's business community. Users are allowed an unrestricted number of calls for an overall rate rather than being charged on a per-call basis. There are two distinct services: outward WATS, with calls dialed by customers and inward WATS, where the customer pays a rate to receive incoming calls (like the "800" free toll calls offered by many companies).

In order to reduce the impact on large volume customers, including those who use WATS lines for computer messages, AT & T said interim rates would delay the full effect for six months after Aug. 1.