The Federal Communications Commission has forced a delay in the scheduled start of the new Post Office electronic mail service.

The chief of the FCC's Common Carrier Bureau yesterday rejected Western Union's proposed tariff for its portion of the Post Office's Electronic Computer Originated Mail (ECOM) service.

The Bureau said that Western Union has not yet supplied enough economic data to support its proposed charges to the post office for lines and switching equipment Western Union will provide as part of ECOM.

The tariff was scheduled to go into effect on Sunday for the new service, which provides guaranteed two-day delivery for computer-originiated messages sent directly from high-volume users to local post offices.

But the bureau, in a decision that will be reviewed by the entire commission, said that Western Union failed to explain or justify its "basis of ratemaking," did not submit underlying studies needed, and did not provide essential cost information.

Further, the bureau decided it would reserve approval of the tariff until another FCC inquiry into the extent of the commission's jurisdiction over the overall ECOM service is completed.

There has been considerable controversy over the roles of the FCC and the Postal Rates Commission over this new service, which was first proposed by the post office in June 1978.