The Supreme Court ruled unanimously Tuesday that states may tax some income from oil and natural gas extracted from the outer continental shelf.

The justices upheld an Iowa tax, challenged by Shell Oil Co., that is imposed on that portion of a company's income derived from doing business in Iowa.Justice Thurgood Marshall, writing for the court, said the state levy does not violate a 1953 federal law authorizing exploration and production of outer continental shelf oil and gas.

"We reject Shell's argument that Congress intended . . . to prohibit the inclusion, in a constitutionally permissible apportionment formula, of income from Outer Contintental Shelf oil and gas," Marshall said.

He said the law only prohibits a state adjacent to such offshore land from taxing that land directly. Nothing in the law bans a state from imposing a tax such as Iowa's limited to that portion of a company's income derived from selling outer continental shelf oil and gas within that state.

The outer continental shelf is underwater land more than three miles off the shore of any state. The land is owned by the federal government and leased to private companies.

Today's ruling upheld a 1987 decision by the Iowa Supreme Court.

The Reagan administration said it agreed with the state court ruling and urged the U.S. Supreme Court to decide in Iowa's favor.

The Justice Department said the Iowa case presented a good opportunity for the high court to resolve the dispute over a state's authority to tax a multi-state activity.

The Iowa tax on multi-state transactions uses a formula called the unitary method to determine what a company must pay the state.

The unitary method, used by many states, allows Iowa to claim as its share of taxable income the fraction of corporate income that equals the company's gross sales in Iowa divided by the company's total sales.