The Federal Communications Commission yesterday rejected a request by American Telephone & Telegraph Co. to more than double the fees its long-distance competitors have to pay to gain access to the Bell System's local network.

Finding AT&T's request "unlawful," the commission instead agreed to a staff proposal that will increase the monthly charges AT&T's competitors must pay by 50 percent--from $128 a line to $183 a line.

The decision was denounced immediately by AT&T and its chief long-distance competitor, MCI Communications Corp.

AT&T said the rate increase did not go far enough to recover its costs, while MCI said the increase ordered by the commission was about four times cost and thus unfair.

For MCI and other long-distance companies, however, the impact of the rate increase will be minimal because last April, the FCC ordered these companies to begin paying a monthly fee of $178 per line, pending its final decision on AT&T's rate request.

As a result, yesterday's decision will mean that these companies will have to pay $5 more a line a month than they have been paying. Additionally, they will have to pay AT&T a total of $1.5 million to account for the difference in the fees they have been paying for the past five months and the fees the FCC ordered yesterday.

MCI Chairman William McGowan said that MCI will account for the largest share of that $1.5 million sum, paying about $1 million.