A modified oil company divestiture bill was approved by a one-vote margin in a House subcommittee yesterday after its chairman, Rep. Morris K. Udall (D-Ariz), cast a proxy "yea" vote that was later disavowed.

The action cast doubt on the fate of the legislation, which would ban major oil companies from obtaining new federal energy-develpment leases unless they rid themselves of coal or uranium interests.

In his energy and environment sub-committee , Udall broke a tie vote on sending the bill to the full Interior Committee by casting a proxy vote for it from Rep. Bob Eckhardt (D-Tex.).

Eckhardt, who was attending another committee meeting at the time, returned just as Udall's subcommittee was breaking up and told Udall that he had not given him his proxy, an Eckhardt aide said.

"I know you must have made an inadvertent error in voting me for the bill because I have not given a proxy on the bill," Eckhardt wrote Udall later. The Texan noted that he opposes divestiture, and an aide quoted him as saying he would have voted against the bill if he had been there.

The vote to approve the bill was 12 to 11. Eckhardt requested that the error be corrected. To record him against the measure would defeat it, 11 to 12. Udall was unavailable last night to comment on the status of the legislation.

The bill would require an oil company with worldwide daily production of 1.6 million barrels or more to divest itself of coal and uranium operations if it intends to bid for oil and gas leases on federal lands and the outer continental shelf after 1980. This would embrace the eight largest oil companies.